Optical waveplate, method of manufacturing the same, and waveguide device using the same

ABSTRACT

An optical waveplate includes a polyimide with a film thickness of 20 mu m or less.

This is a divisional of Ser. No. 09/111,549, filed Jul. 7, 1998 which isa divisional of Ser. No. 08/840,139 filed Apr. 11, 1997, now U.S. Pat.No. 5,901,259 which is a divisional of Ser. No. 08/747,193 filed Nov.12, 1996, now U.S. Pat. No. 5,694,496 which is a continuation of Ser.No. 08/645,920 filed May 14, 1996, now abandoned which is a continuationof Ser. No. 08/237,109 filed May 3, 1994, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to an optical waveplate for use in anoptical communication system, a method of manufacturing the opticalwaveplate, and a waveguide device using the optical waveplate.

Recently, various methods have been proposed for transmission of a largequantity of information stably and inexpensively. An opticalcommunication system is one of these methods. A representative exampleof this optical communication system is a method (wavelength divisionmultiplexing system) in which light components having a plurality ofwavelengths and carrying their respective signals are multiplexed intosingle light by a multiplexer and transmitted to a remote place throughan optical fiber. The light is demultiplexed into the light componentswith their original wavelengths by a demultiplexer upon reception,thereby detecting the individual signals. This method can increase thecommunication capacity in proportion to the multiplexing number ofwavelengths and is therefore a very effective method of increasing thecapacity. This method also can reduce the load on hardware in an opticalcommunication network connecting a number of points and makes a moreadvanced network arrangement possible by using a combination of aplurality of light wavelengths, multiplexers, and demultiplexers inaddition to spatial wiring.

A system of the above sort requires a light source which oscillates at aplurality of wavelengths, and a multi/demultiplexer formultiplexing/demultiplexing light. As the multi/demultiplexer, a deviceusing PLCs (Planar Lightwave Circuits) consisting of optical waveguidesformed on a substrate has been developed as the most realistic devicefrom the point of view of a small size, a light weight, and a highreliability. Of these PLCs, a silica-based PLC fabricated by depositinga silica glass film on a silicon substrate is expected as a practicaloptical component, since it has a small optical loss and consequently ahigh stability against disturbance such as heat or vibrations.

The most serious problem in putting the silica-based PLC into practicaluse is its polarization dependence. That is, as mentioned above, thissilica-based PLC is manufactured by depositing a glass film on a siliconsubstrate. Therefore, the difference in thermal expansion coefficientbetween the glass film and the silicon substrate makes a stress appliedon an optical waveguide in a direction parallel to the surface of thesubstrate differ from that in a direction perpendicular to the substratesurface. Consequently, the refractive index of the optical waveguide inthe direction parallel to the surface of the silicon substrate becomesdifferent from that in the direction perpendicular to the substratesurface. This is termed "waveguide birefringence". When, for example, anasymmetrical Mach-Zender interferometer is constituted by thesilica-based PLC, this waveguide birefringence gives rise to a problemthat the optical path length difference (a difference in refractiveindex x physical length) of an arm constituting the interferometerchanges depending on the polarizing direction of light. Consequently,the device characteristics change in accordance with the polarized stateof light. This makes it impossible to apply the device to a system usinga single-mode fiber.

This problem of the polarization dependence of the PLC caused by thewaveguide birefringence is not inherent in a silica-based glasswaveguide. That is, all waveguides currently manufactured have thisproblem because they also have waveguide birefringence, although thedegrees of waveguide birefringence differ from one another. Examples ofthe waveguides are a titanium in-diffused LiNbO₃ waveguide, aproton-exchanged LiNbO₃ waveguide, an ion-exchanged glass waveguide, asemiconductor waveguide, a polycarbonate waveguide, a polyimidewaveguide, a silicone resin waveguide, and an epoxy resin waveguide.

As a method for compensating for the birefringence of a silica-basedoptical waveguide, a method of mounting amorphous silicon on top of awaveguide and using the resultant stress is known. This method, however,requires some additional steps, such as a step of mounting amorphoussilicon and a step of trimming the amorphous silicon by using a laser,after a sample is formed, in order to finely adjust the stress. Inaddition, since it is difficult to compensate for the waveguidebirefringence across a wide area, individual waveguides must be spacedapart from one another. It is, therefore, impossible to apply thismethod to waveguides integrated at a high density. As described above,the method using the stress of amorphous silicon has several practicalproblems.

Takahashi et al., on the other hand, have developed a method ofeliminating the polarization dependence of the PLC by inserting a halfwaveplate consisting of a rock crystal at the center of an opticalcircuit of an arrayed-waveguide grating-type wavelengthmulti/demultiplexer such that the optical principal axis of the halfwaveplate forms an angle of 45° with a substrate. (Hiroshi Takahashi etal., "Optics Letters," Vol. 17, No. 7, pp. 499-501 (1992)). Takahashi etal. have also pointed out in Japanese Patent Prepublication No. 4-241304that this method is also effective in eliminating the polarizationdependence of a Mach-Zender interferometer, a ring resonator, adirectional coupler, and a phase modulator. This method of eliminatingthe polarization dependence of an optical circuit by inserting arock-crystal half waveplate at the center of the optical circuitrealizes a high reliability for long periods of time, has simplemanufacturing steps, and can be applied to all waveguides in addition toa silica-based glass waveguide. Therefore, the method is very effectivecompared to the above-mentioned method by which amorphous silicon ismounted.

A rock crystal has a high heat resistance, a high humidity resistance,and a high precision processability and shows stable opticalcharacteristics. Therefore, a PLC incorporating a rock-crystal halfwaveplate has a high reliability. However, this method has a largedrawback; that is, since there is no light-confining structure in thehalf waveplate and in a groove for receiving the half waveplate, lightpropagating through the waveguide is radiated from these portions,resulting in loss of light. According to the report by Takahashi et al.,an excess loss of 5 dB is produced when a half waveplate consisting of arock crystal is inserted into a 100-μm wide groove formed in a waveguidewith a specific refractive index difference of 0.75%. This value isextremely large compared to a loss of 2 to 3 dB of the PLC itself.Consequently, it has been impossible to apply the method to actual PLCsfrom the point of view of the optical loss.

To obtain a PLC incorporating an optical waveplate as a highly practicalcomponent, it is important to decrease the excess loss produced byinsertion of the waveplate to 0.5 dB or less (i.e., to reduce thedecrease in the light quantity to 10% or less).

FIG. 1 shows the result of simulation of the excess loss performed byassuming that a light beam emitted from the end face of an opticalwaveguide is a Gaussian beam. This characteristic curve illustrated inFIG. 1 shows that the excess loss is reduced to 0.3 dB or less when thefilm thickness of an optical waveplate is 20 μm or less.

In a practical case, however, a loss of about 0.1 to 0.2 dB isunavoidable because of Fresnel reflection or scattering at the end faceof a waveplate. When this fact is taken into consideration, therefore,the film thickness of an optical waveplate must be 20 μm or smaller inorder to reduce the excess loss as a result of insertion of a waveplateto 0.5 dB or less. To manufacture a half waveplate, with a wavelength(1.3 μm, 1.55 μm) currently used in long-distance optical communication,to have a film thickness of 20 μm or smaller, the material of thewaveplate is required to have an in-plane birefringence greater than atleast 0.03. A rock-crystal half waveplate brings about a large excessloss as described above because its thickness is as large as 91 μm. Thislarge thickness results from a small birefringence of a rock crystal of0.0085 at a wavelength of 1.3 μm. The use of a material having a largebirefringence makes it possible to manufacture a thin waveplate, andthis results in a decreased excess loss. Calcite and titanium oxide areknown as inorganic single-crystal materials, other than a rock crystal,having a large birefringence; both calcite and titanium oxide have abirefringence larger than that of a rock crystal. However, the rough ofcalcite is expensive, and the thickness of a half waveplate consistingof calcite becomes as very small as 4 μm because the birefringence ofcalcite is large, 0.16, at a wavelength of 1.3 μm. Since the hardness ofcalcite is low (Mohs hardness: 2), it is very difficult to processcalcite to have this small thickness. Even if calcite can be thusprocessed, the product must be handled with enough care. On the otherhand, the refractive index of titanium oxide is 2.62 to 2.90, which islargely different from those of silica and other optical waveguidematerials. Therefore, when a waveplate consisting of titanium oxide isinserted into an optical waveguide, a loss caused by Fresnel reflectionat the end face of the waveguide is large. Consequently, the effect ofdecreasing the thickness of a waveplate becomes insignificant. For thereasons discussed above, neither calcite nor titanium oxide is asuitable material to be inserted into a lightwave circuit.

In order that a waveguide device in which a half waveplate is insertedbe used in practice, the heat resistance and the humidity resistance ofthe waveplate and the ease in handling the waveplate are also importantfactors. For example, a waveguide device fabricated on a singlesubstrate is used not only as a single component by itself but also asan "optical and electronic hybrid interconnection" in combination withother lightwave circuits and electric circuits fabricated on the samesubstrate. The fabrication of these photonic components involves asoldering step performed at about 260° C. and a step performed at atemperature which temporarily exceeds 300° C. Therefore, all thematerials used in the fabrication are required to have a heat resistanceof about 350° C.

An amorphous polymer plastic material is known as a material whichproduces a birefringence. Representative examples of such a polymermaterial are polycarbonate and polyvinyl alcohol. These materialsproduce an in-plane birefringence when films consisting of the materialsare drawn. In practice, large retardation plates for use inliquid-crystal displays are manufactured by using these polymermaterials. Retardation plates consisting of polystyrene, a cellulosederivative, polyvinyl chloride, polypropylene, an acrylic polymer,poly(amic acid), polyester, and an ethylene-vinyl acetate copolymersaponified material are also known. However, the polyvinyl alcohol-basedmaterial and the cellulose derivative-based material have a low humidityresistance, and the polypropylene-based material is unsatisfactory intoughness. The acryl-based material is difficult to draw because itsmechanical strength in the form of a film is low. Thepolycarbonate-based material is poor in chemical resistance.

The polyvinyl chloride material and the polystyrene-based material areunsatisfactory particularly in heat resistance and are thereforeinadequate for the purpose of the present invention. Although thepoly(amic acid)-based material and the polyester-based material areconsidered to have a relatively high heat resistance, none of thesematerials has a heat resistance of 300° C. or higher which is requiredfor waveguide devices. Also, a waveplate made from any of these organicpolymer materials is reduced in birefringence due to activation ofmolecular motion even at a temperature lower than its softening point(glass transition temperature). This largely degrades thecharacteristics as a waveplate. In addition, not a few of these organicpolymer materials have a saturation water absorption of 2 to 3%. Since,however, water molecules strongly absorb light with opticalcommunication wavelengths to increase the loss, the material to be usedas a waveplate must have as low a water absorption as possible.

As discussed above, it is difficult to manufacture waveplates that canbe incorporated in optical waveguides by using any of the conventionallyknown polymer materials.

In summary, the problems of the conventional optical waveplatetechniques are as follows. That is, for waveplates using inorganicsingle-crystal materials, no material having an appropriatebirefringence and refractive index by which a waveplate can beincorporated in a waveguide device is available. In addition, thesematerials are difficult to process and expensive. On the other hand,waveplates consisting of plastic materials have problems in the heatresistance, humidity resistance, and mechanical strength of a material,and in the stability of in-plane birefringence.

SUMMARY OF THE INVENTION

It is, therefore, a principal object of the present invention to providean optical waveplate which can be readily manufactured and processed,has high heat resistance, humidity resistance, flexibility, andmechanical strength, and also has a small film thickness, a method ofmanufacturing this optical waveplate, and a waveguide device using thewaveplate.

It is another object of the present invention to provide a waveguidedevice having a sufficient optical transparency.

To achieve the above objects, the present inventors have focusedattention on a polyimide optical material which is applicable to opticalwaveguides, in view of the fact that the existing plastic opticalmaterials are unsatisfactory in heat resistance and humidity resistance.

As for this polyimide optical material, "Macromolecules" [T. Matsuura etal., Vol. 24, pp. 5,001-5,005, 1991 and T. Matsuura et al., Vol. 25, pp.3,540-3,545, 1992] have already reported that polyimide films having aheat resistance of 300° C. or higher, a low water absorption of 0.7% orlower, and a high optical transparency can be obtained by synthesizingvarious fluorinated polyimides by using2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl as a diamine component.T. Matsuura et al. have also reported in Elec. Lett. Vol. 29, No. 24,pp. 2,107-2,109 that good optical waveguides for near-infrared light canbe formed by using, as a core and a cladding, a polyimide synthesized byusing a diamine and two different types of tetracarboxylic dianhydrides.In addition, in "Macromolecules" [T. Matsuura et al., Vol. 25, pp.5,858-5,860, 1992], S. Ando et al. have reported a perfluorinatedpolyimide having no light absorption peak in the entire opticalcommunication wavelength region (wavelength 1.0 to 1.7 μm) and having aheat resistance and a low water absorption equivalent to those offluorinated polyimides. This makes it possible to provide a plasticoptical material with a very small loss even in a wavelength band inwhich heat-resistant plastic materials are conventionally difficult touse because they have absorption peaks inherent in their molecularstructures. It is also found that polyimides can be readily processedand handled because of their high flexibility and are superior to otherorganic polymer materials in toughness.

The gist of the present invention is to apply these characteristicfeatures of the polyimide optical material to optical waveplates.

The basic concept of the present invention is to form an opticalwaveplate by using a polyimide having a film thickness of 20 μm orsmaller. An optical waveplate with this arrangement is formed bythermally imidizing a poly(amic acid) solution, which is synthesizedfrom a tetracarboxylic acid or its derivative and a diamine, to have afilm thickness of 20 μm or less. In this case, the formed film issubjected to uniaxial drawing or equivalent strain processing.

Although details of examples of this strain processing will be discussedlater as the examples of the present invention, the processing will bebriefly explained below. That is, a poly(amic acid) solution synthesizedfrom a tetracarboxylic acid or its derivative and a diamine is coated ona substrate and dried for a short time period. Thereafter, the resultantfilm is peeled from the substrate with the solvent contained in thefilm, and uniaxially drawn. The film is then fixed to a metal frame orthe like and thermally imidized. In another example, a poly(amic acid)film is thermally imidized while it is uniaxially drawn. In stillanother example, a poly(amic acid) film is thermally imidized while itis fixed only in a uniaxial drawing direction by a metal frame or thelike. In still another example, a poly(amic acid) solution is coated ona substrate having an anisotropy of thermal expansion coefficient in itsplane, and the resultant material is thermally imidized. In stillanother example, a polyimide film is uniaxially drawn at a hightemperature of 300° C. or higher. In still another example, a polyimidefilm is thermally treated at a temperature of 300° C. or higher.

A waveguide device is constituted by using an optical waveguide formedon a substrate and the polyimide waveplate characterized by the presentinvention. As an example, a waveguide device is constituted by insertingthe optical waveplate of the present invention into an optical waveguidesuch that the waveplate is either perpendicular to or inclined from thelongitudinal direction of the waveguide. Alternatively, a half waveplateaccording to the present invention is inserted into a waveguide suchthat the optical principal axis of the half waveplate makes an angle of45° with a waveguide substrate.

Still another waveguide device characterized by the present inventioncomprises an optical waveguide formed on a substrate and a polarizationconvertor consisting of a polyimide optical waveplate arranged in themiddle of the optical path of the waveguide. This polarization convertorconsisting of the polyimide optical waveplate converts horizontalpolarization (TE mode: light having an electric field component in aplane parallel to a substrate) contained in guided light into verticalpolarization (TM mode light having an electric field component in aplane perpendicular to a substrate) and vice versa. With thisarrangement, the dependence of the waveguide device on polarization canbe eliminated. Eliminating the polarization dependence by replacing thehorizontal polarization with the vertical polarization and vice versa byinserting a half waveplate in the middle of the optical path isidentical in principle with the method disclosed in Japanese PatentPrepublication No. 4-241304 mentioned earlier. The characteristicfeature of the present invention, however, is that an excess loss causedby insertion of a waveplate is largely decreased by applying a polyimideoptical waveplate with a film thickness of 20 μm or less to a waveguidedevice.

Still another waveguide device characterized by the present inventioncomprises an optical waveguide formed on a substrate and a polyimideoptical waveplate. This waveplate inserted into the waveguide consistsof a novel polarization beam splitter as a quarter waveplate. Thispolarization beam splitter is so inserted that its optical principalaxis is either perpendicular or parallel to the waveguide substrate.Although the principle of operation of this invention will be describedin detail later by way of its examples, this quarter waveplate is notused as a polarization convertor but used to cause the horizontalpolarization of guided light to have an optical path length longer orshorter by a quarter wavelength than that for the vertical polarization.

Still another waveguide device characterized by the present inventioncomprises magnetic and nonmagnetic waveguides formed on a substrate anda polyimide optical waveplate. This waveplate inserted into thewaveguide consists of a novel circulator as a half waveplate. Thiscirculator is so inserted that its optical principal axis forms an angleof 22.5° or 67.5° with the waveguide substrate. Although details of theoperating principle of this invention will also be described later byway of the examples, this half waveplate is used to rotate thepolarizing direction of guided light through 45° or 135°.

Still another waveguide device characterized by the present inventioncomprises an optical waveguide formed on a substrate and a polyimideoptical waveplate. This waveplate is in tight contact with the end faceof the waveguide so as to be perpendicular to or inclined from thelongitudinal direction of the waveguide. In addition, a reflecting coatis formed on the side of the waveplate not in contact with the end faceof the waveguide. This allows a single device to achieve both the effectobtained by the waveplate and the reflection of light. This waveplatecan be a quarter waveplate. In this case, the optical principal axis ofthis quarter waveplate is so arranged as to make an angle of 45° withthe waveguide substrate. Although the operating principle of thisinvention will also be described in detail later by way of the examples,the quarter waveplate and the reflecting coat formed on it are used forthe purposes of reflecting guided light and rotating the polarizingdirection of the light through 90°.

In still another waveguide device characterized by the presentinvention, a plurality of several different types of waveguide devicesas described above are formed on the same substrate and coupled to eachother through optical waveguides.

Note that a method of obtaining a birefringence in the plane of apolyimide film is described in K. Nakagawa, "J. Appl. Polymer Sci.,"Vol. 41, pp. 2,049-2,058, 1990. In this method, a film consisting of apoly(amic acid) synthesized from a pyromellitic dianhydride and4,4'-diaminodiphenylether is thermally imidized up to 160° C. under atensile stress and then thermally treated up to 350° C. By this method,drawing of a maximum of 83% is possible, and a polyimide film having alarge in-plane birefringence of approximately 0.18 (wavelength 0.633 μm)can be obtained when drawing of 30% or more is done. However, thisliterature does not mention a method of controlling the birefringenceand the film thickness required to apply polyimides to waveplates.

The present inventors, therefore, have performed uniaxial drawing forfilms consisting of poly(amic acid)s and polyimides, which aresynthesized by combining various acid anhydrides as derivatives oftetracarboxylic acids with various diamines, by using several differentmethods. Consequently, it is found that the anisotropy of a refractiveindex (birefringence) appears in the plane of a film in each and everycase. Thereafter, the present inventors have made extensive studies on amethod of controlling the in-plane birefringence and the film thicknessafter thermal imidization, and completed the optical waveplatesaccording to the present invention and the method of manufacturing thewaveplates.

Consequently, the present inventors have completed the waveguide devicesaccording to the present invention by incorporating the various opticalwaveplates obtained by the above method in waveguide devices eachcomprising one or more optical waveguides with birefringence formed on asubstrate.

FIGS. 2A and 2B are views each for explaining the effect of drawing fora refractive index ellipsoid representing the refractive indexanisotropy of polyimide films. FIG. 2A illustrates the refractive indexellipsoid of a polyimide film not subjected to the drawing, and FIG. 2Billustrates the refractive index ellipsoid of a polyimide film subjectedto the drawing. When no drawing is performed, a refractive indexanisotropy (birefringence) is found in a direction perpendicular to theplane of the film, but no refractive index anisotropy is found in thedirection of the plane (n_(TE1) =n_(TE2)). After the drawing isperformed, however, the birefringence is found not only in the directionperpendicular to the plane but also in the direction of the plane(n_(TE1) =n_(TE2)), since the molecular chains orient in the drawingdirection. In the present invention, of n_(TE1) and n_(TE2)perpendicular to each other, n_(TE1) which has a larger refractive indexand the same direction as the drawing direction is defined as theoptical principal axis. This axis is sometimes also called a slow axis.If a value (retardation) calculated by multiplying the in-planebirefringence (Δn: n_(TE1) -n_(TE2)) by the film thickness (d) is inagreement with a half or quarter of the wavelength of a light beam, thefilm can be used as a half or quarter waveplate. The film can also beused as a waveplate of a higher order by controlling the in-planebirefringence and the film thickness.

Examples of the tetracarboxylic acid, and an acid anhydride, an acidchloride, and an ester as derivatives of the tetracarboxylic acid foruse in the present invention are as follows. The names enumerated beloware names as tetracarboxylic acids. Examples are:

pyromellitic acid,

trifluoromethylpyromellitic acid,

pentafluoroethylpyromellitic acid,

bis{3,5-di(trifluoromethyl)phenoxy}pyromellitic acid,

2,3,3',4'-biphenyltetracarboxylic acid,

3,3',4,4'-tetracarboxydiphenylether,

2,3',3,4'-tetracarboxydiphenylether,

3,3',4,4'-benzophenonetetracarboxylic acid,

2,3,6,7-tetracarboxynaphthalene,

1,4,5,7-tetracarboxynaphthalene,

1,4,5,6-tetracarboxynaphthalene,

3,3',4,4'-tetracarboxydiphenylmethane,

3,3',4,4'-tetracarboxydiphenylsulfone,

2,2-bis(3,4-dicarboxyphenyl)propane,

2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane,

5,5'-bis(trifluoromethyl)-3,3',4,4'-tetracarboxybiphenyl,

2,2',5,5'-tetrakis(trifluoromethyl)-3,3',4,4'-tetracarboxybiphenyl,

5,5'-bis(trifluoromethyl)-3,3',4,4'-tetracarboxydiphenylether,

5,5'-bis(trifluoromethyl)-3,3',4,4'-tetracarboxybenzophenone,

bis{trifluoromethyl)dicarboxyphenoxy}benzene,

bis{(trifluoromethyl)dicarboxyphenoxy}(trifluoromethyl)b enzene,

bis(dicarboxyphenoxy)(trifluoromethyl)benzene,

bis(dicarboxyphenoxy)bis(trifluoromethyl)benzene,

bis(dicarboxyphenoxy)tetrakis(trifluoromethyl)benzene,

3,4,9,10-tetracarboxyperylene,

2,2-bis{4-(3,4-dicarboxyphenoxy)phenyl}propane,

butanetetracarboxylic acid,

cyclopentanetetracarboxylic acid,

2,2-bis{4-(3,4-dicarboxyphenoxy)phenyl}hexafluoropropane,

bis{(trifluoromethyl)dicarboxyphenoxy}biphenyl,

bis{(trifluoromethyl)dicarboxyphenoxy}bis(trifluoromethy 1)biphenyl,

bis{(trifluoromethyl)dicarboxyphenoxy}diphenylether,

bis(dicarboxyphenoxy)bis(trifluoromethyl)biphenyl,

bis(3,4-dicarboxyphenyl)dimethylsilane,

1,3-bis (3,4-dicarboxyphenyl)tetramethyldisiloxane,

1,4-bis(3,4-dicarboxytrifluorophenoxy)tetrafluorobenzene,

1,4-bis(3,4-dicarboxytrifluorophenoxy)octafluorobiphenyl,

1,4-difluoropyromellitic acid,

1-trifluoromethyl-4-fluoropyromellitic acid,

1,4-di(trifluoromethyl)pyromellitic acid,

1-pentafluoroethyl-4-fluoropyromellitic acid,

1-pentafluoroethyl-4-trifluoromethylpyromellitic acid,

1,4-di(pentafluoroethyl)pyromellitic acid,

1-pentafluorophenyl-4-fluoropyromellitic acid,

1-pentafluorophenyl-4-trifluoromethylpyromellitic acid,

1-pentafluorophenyl-4-pentafluoroethylpyromellitic acid,

1,4-di(pentafluorophenyl)pyromellitic acid,

1-trifluoromethoxy-4-fluoropyromellitic acid,

1-trifluoromethoxy-4-trifluoromethylpyromellitic acid,

1-trifluoromethoxy-4-pentafluoroethylpyromellitic acid,

1-trifluoromethoxy-4-pentafluorophenylpyromellitic acid,

1,4-di(trifluoromethoxy)pyromellitic acid,

1-pentafluoroethoxy-4-fluoropyromellitic acid,

1-pentafluoroethoxy-4-trifluoromethylpyromellitic acid,

1-pentafluoroethoxy-4-pentafluoroethylpyromellitic acid,

1-pentafluoroethoxy-4-pentafluorophenylpyromellitic acid,

1-pentafluoroethoxy-4-trifluoromethoxypyromellitic acid,

1,4-di(pentafluoroethoxy)pyromellitic acid,

1-pentafluorophenoxy-4-fluoropyromellitic acid,

1-pentafluorophenoxy-4-trifluoromethylpyromellitic acid,

1-pentafluorophenoxy-4-pentafluoroethylpyromellitic acid,

1-pentafluorophenoxy-4-pentafluorophenylpyromellitic acid,

1-pentafluorophenoxy-4-trifluoromethoxypyromellitic acid,

1-pentafluorophenoxy-4-pentafluoroethoxypyromellitic acid,

1,4-di(pentafluorophenoxy)pyromellitic acid,

hexafluoro-3,3',4,4'-biphenyltetracarboxylic acid,

hexafluoro-3,3',4,4'-biphenylethertetracarboxylic acid,

hexafluoro-3,3',4,4'-benzophenonetetracarboxylic acid,

bis(3,4-dicarboxytrifluorophenyl)sulfone,

bis(3,4-dicarboxytrifluorophenyl)sulfide,

bis(3,4-dicarboxytrifluorophenyl)difluoromethane,

1,2-bis(3,4-dicarboxytrifluorophenyl)tetrafluoroethane,

2,2-bis(3,4-dicarboxytrifluorophenyl)hexafluoropropane,

1,4-bis(3,4-dicarboxytrifluorophenyl)tetrafluorobenzene,

3,4-dicarboxyfluorophenyl-3',4'-dicarboxytrifluorophenoxy-difluoromethane,

bis(3,4-dicarboxytrifluorophenoxy)difluoromethane,

1,2-bis(3,4-dicarboxytrifluorophenoxy)tetrafluoroethane,

2,2-bis(3,4-dicarboxytrifluorophenoxy)hexafluoropropane,

1,4-bis(3,4-dicarboxytrifluorophenoxy)tetrafluorobenzene,

2,3,6,7-tetracarboxy-tetrafluoronaphthalene,

2,3,6,7-tetracarboxy-hexafluoroanthracene,

2,3,6,7-tetracarboxy-hexafluorophenanthrene,

2,3,6,7-tetracarboxy-tetrafluorobiphenylene,

2,3,7,8-tetracarboxy-tetrafluorodibenzofuran,

2,3,6,7-tetracarboxy-tetrafluoroanthraquinone,

2,3,6,7-tetracarboxy-pentafluoroanthrone,

2,3,7,8-tetracarboxy-tetrafluorophenoxathiin,

2,3,7,8-tetracarboxy-tetrafluorothianthrene, and

2,3,7,8-tetracarboxy-tetrafluorodibenzo[b,e]1,4dioxane.

Examples of the diamine for use in the present invention are:

m-phenylenediamine,

2,4-diaminotoluene,

2,4-diaminoxylene,

2,4-diaminodurene,4-(1H,1H,11H-eicosafluoroundecanoxy)-1,3-diaminobenzene,

4-(1H,1H-perfluoro-1-butanoxy)-1,3-diaminobenzene,

4-(1H,1H-perfluoro-1-heptanoxy)-1,3-diaminobenzene,

4-(1H,1H-perfluoro-1-octanoxy)-1,3-diaminobenzene,

4-pentafluorophenoxy-1,3-diaminobenzene,

4-(2,3,5,6-tetrafluorophenoxy)-1,3-diaminobenzene,

4-(4-fluorophenoxy)-1,3-diaminobenzene,

4-(1H,1H,2H,2H-perfluoro-1-dodecanoxy)-1,3-diaminobenzene,

p-phenylenediamine,

2,5-diaminotoluene,

2,3,5, 6-tetramethyl-p-phenylenediamine,

2,5-diaminobenzotrifluoride,

bis(trifluoromethyl)phenylenediamine,

diaminotetra(trifluoromethyl)benzene,

diamino(pentafluoroethyl)benzene,

2,5-diamino(perfluorohexyl)benzene,

2,5-diamino(perfluorobutyl)benzene,

benzidine,

2,2'-dimethylbenzidine,

3,3'-dimethylbenzidine,

3,3'-dimethoxybenzidine,

2,2'-dimethoxybenzidine,

3,3',5,5'-tetramethylbenzidine,

3,3'-diacetylbenzidine,

2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl,

3,3'-bis(trifluoromethyl)-4,4'-diaminobiphenyl,

4,4'-diaminodiphenylether,

4,4'-diaminodiphenylmethane,

4,4'-diaminodiphenylsulfone,

2,2'-bis(p-aminophenyl)propane,

3,3'-dimethyl-4,4'-diaminodiphenylether,

3,3'-dimethyl-4,4'-diaminodiphenylmethane,

1,2-bis(anilino)ethane,

2,2-bis(p-aminophenyl)hexafluoropropane,

1,3-bis(anilino)hexafluoropropane,

1,4-bis(anilino)octafluorobutane,

1,5-bis(anilino)decafluoropentane,

1,7-bis(anilino)tetradecafluoroheptane,

2,2'-bis(trifluoromethyl)-4,4'-diaminodiphenylether,

3,3'-bis(trifluoromethyl)-4,4'-diaminodiphenylether,

3,3',5,5'-tetrakis(trifluoromethyl)-4,4'-diaminodiphenylether,

3,3'-bis(trifluoromethyl)-4,4'-diaminobenzophenone,

4,4"-diamino-p-terphenyl,

1,4-bis(p-aminophenyl)benzene,

p-bis(4-amino-2-trifluoromethylphenoxy)benzene,

bis(aminophenoxy)bis(trifluoromethyl)benzene,

bis(aminophenoxy)tetrakis(trifluoromethyl)benzene,

4,4'"-diamino-p-quaterphenyl,

4,4'-bis(p-aminophenoxy)biphenyl,

2,2-bis{4-(p-aminophenoxy)phenyl}propane,

4,4'-bis(3-aminophenoxyphenyl)diphenylsulfone,

2,2-bis{4-(4-aminophenoxy)phenyl}hexafluoropropane,

2,2-bis{4-(3-aminophenoxy)phenyl}hexafluoropropane,

2,2-bis{4-(2-aminophenoxy)phenyl}hexafluoropropane,

2,2-bis{4-(4-aminophenoxy)-3,5-dimethylphenyl}hexafluoropropane,

2,2-bis{4-(4-aminophenoxy)-3,5-ditrifluoromethylphenyl}hexafluoropropane,

4,4'-bis(4-amino-2-trifluoromethylphenoxy)biphenyl,

4,4'-bis(4-amino-3-trifluoromethylphenoxy)biphenyl,

4,4'-bis(4-amino-2-trifluoromethylphenoxy)diphenylsulfone,

4,4'-bis(3-amino-5-trifluoromethylphenoxy)diphenylsulfone,

2,2-bis{4-(4-amino-3-trifluoromethylphenoxy)phenyl}hexafluoropropane,

bis{(trifluoromethyl)aminophenoxy}biphenyl,

bis[{(trifluoromethyl)aminophenoxy}phenyl]hexafluoropropane,

diaminoanthraquinone,

1,5-diaminonaphthalene,

2,6-diaminonaphthalene,

bis[{2-(aminophenoxy)phenyl}hexafluoroisopropyl]benzene,

bis(2,3,5,6-tetrafluoro-4-aminophenyl)ether,

bis(2,3,5,6-tetrafluoro-4-aminophenyl)sulfide,

1,3-bis(3-aminopropyl)tetramethyldisiloxane,

1,4-bis(3-aminopropyldimethylsilyl)benzene,

bis(4-aminophenyl)diethylsilane,

tetrafluoro-1,2-phenylenediamine,

tetrafluoro-1,3-phenylenediamine,

tetrafluoro-1,4-phenylenediamine,

hexafluoro-1,5-diaminonaphthalene,

hexafluoro-2,6-diaminonaphthalene,

3-trifluoromethyl-trifluoro-1,2-phenylenediamine,

4-trifluoromethyl-trifluoro-1,2-phenylenediamine,

2-trifluoromethyl-trifluoro-1,3-phenylenediamine,

4-trifluoromethyl-trifluoro-1,3-phenylenediamine,

5-trifluoromethyl-trifluoro-1,3-phenylenediamine,

2-trifluoromethyl-trifluoro-1,4-phenylenediamine,

3,4-bis(trifluoromethyl)-difluoro-1,2-phenylenediamine,

3,5-bis(trifluoromethyl)-difluoro-1,2-phenylenediamine,

2,4-bis(trifluoromethyl)-difluoro-1,3-phenylenediamine,

4,5-bis(trifluoromethyl)-difluoro-1,3-phenylenediamine,

4,6-bis(trifluoromethyl)-difluoro-1,3-phenylenediamine,

2,3-bis(trifluoromethyl)-difluoro-1,4-phenylenediamine,

2,5-bis(trifluoromethyl)-difluoro-1,4-phenylenediamine,

3,4,5-tris(trifluoromethyl)-fluoro-1,2-phenylenediamine,

3,4,6-tris(trifluoromethyl)-fluoro-1,2-phenylenediamine,

2,4,5-tris(trifluoromethyl)-fluoro-1,3-phenylenediamine,

2,4,6-tris(trifluoromethyl)-fluoro-1,3-phenylenediamine,

4,5,6-tris(trifluoromethyl)-fluoro-1,3-phenylenediamine,

tetrakis(trifluoromethyl)-1,2-phenylenediamine,

tetrakis(trifluoromethyl)-1,3-phenylenediamine,

tetrakis(trifluoromethyl)-1,4-phenylenediamine,

3-pentafluoroethyl-trifluoro-1,2-phenylenediamine,

4-pentafluoroethyl-trifluoro-1,2-phenylenediamine,

2-pentafluoroethyl-trifluoro-1,3-phenylenediamine,

4-pentafluoroethyl-trifluoro-1,3-phenylenediamine,

5-pentafluoroethyl-trifluoro-1,3-phenylenediamine,

2-pentafluoroethyl-trifluoro-1,4-phenylenediamine,

3-trifluoromethoxy-trifluoro-1,2-phenylenediamine,

4-trifluoromethoxy-trifluoro-1,2-phenylenediamine,

2-trifluoromethoxy-trifluoro-1,3-phenylenediamine,

4-trifluoromethoxy-trifluoro-1,3-phenylenediamine,

5-trifluoromethoxy-trifluoro-1,3-phenylenediamine,

2-trifluoromethoxy-trifluoro-1,4-phenylenediamine,

3,3'-diamino-octafluorobiphenyl,

3,4'-diamino-octafluorobiphenyl,

4,4'-diamino-octafluorobiphenyl,

2,2'-bis(trifluoromethyl)-4,4'-diaminohexafluorobiphenyl,

3,3'-bis(trifluoromethyl)-4,4'-diaminohexafluorobiphenyl,

bis(3-amino-tetrafluorophenyl)ether,

3,4'-diamino-octafluorobiphenylether,

bis(4-amino-tetrafluorophenyl)ether,

3,3'-diamino-octafluorobenzophenone,

3,4'-diamino-octafluorobenzophenone,

4,4'-diamino-octafluorobenzophenone,

bis(3-amino-tetrafluorophenyl)sulfone,

3,4'-diamino-octafluorobiphenylsulfone,

bis(4-amino-tetrafluorophenyl)sulfone,

bis(3-amino-tetrafluorophenyl)sulfide,

3,4'-diamino-octafluorobiphenylsulfide,

bis(4-amino-tetrafluorophenyl)sulfide,

bis(4-aminotetrafluorophenyl)difluoromethane,

1,2-bis(4-aminotetrafluorophenyl)tetrafluoroethane,

2,2-bis(4-aminotetrafluorophenyl)hexafluoropropane,

4,4'-diamino-dodecafluoro-p-terphenyl,

4-amino-tetrafluorophenoxy-4'-amino-tetrafluorophenyl -difluoromethane,

bis(4-amino-tetrafluorophenoxy)-difluoromethane,

1,2-bis(4-amino-tetrafluorophenoxy)-tetrafluoroethane,

2,2-bis(4-amino-tetrafluorophenoxy)-hexafluoropropane,

1,4-bis(4-amino-tetrafluorophenoxy)-tetrafluorobenzene,

2,6-diamino-hexafluoronaphthalene,

2,6-diamino-octafluoroanthracene,

2,7-diamino-octafluorophenanthrene,

2,6-diamino-hexafluorobiphenylene,

2,7-diamino-hexafluorobenzofuran,

2,6-diamino-hexafluoroanthraquinone,

2,6-diamino-octafluoroanthrone,

2,7-diamino-hexafluorophenoxathiin,

2,7-diamino-hexafluorothianthrene, and

2,7-diamino-tetrafluorodibenzo[b,e]1,4dioxane.

To achieve a birefringence exceeding 0.03 required to realize apolyimide optical waveplate with a film thickness of 20 μm or smaller,which is characterized by the present invention, by drawing at apractical draw ratio, it is preferable that one or both of thetetracarboxylic acid or its derivative and the diamine have a highlylinear structure in which the skeleton or main chain structure has norotatable bond or has only one rotatable bond. For example, if two ormore rotatable bonds are contained in the skeleton of the diamine (i.e.,if any of an ether group, a thioether group, a methylene group, asulfone group, a carbonyl group, an isopropylidene group, and ahexafluoroisopropylidene group is contained), preferable usable examplesof the tetracarboxylic acid are a pyromellitic acid whose skeletonconsists of one benzene ring, a derivative of this pyromellitic acid inwhich two hydrogen atoms bonded to that benzene ring are substitutedwith another organic substituent or halogen,2,3,3',4'-biphenyltetracarboxylic acid whose skeleton is a biphenylstructure, and a derivative of this 2,3,3',4'-biphenyltetracarboxylicacid in which four hydrogen atoms bonded to the benzene ring of thatbiphenyl structure are substituted with another organic substituent orhalogen. If the skeleton of an acid anhydride contains two or morerotatable bonds, examples of the diamine are preferably a diaminobenzenewhose skeleton consists of one benzene ring, a derivative of thisdiaminobenzene in which four hydrogen atoms bonded to that benzene ringare substituted with another organic substituent or halogen, and aderivative in which the skeleton is a biphenyl structure and some or allof hydrogen atoms bonded to the benzene ring of that biphenyl structureare substituted with another organic group or halogen. As will bepresented later in the examples of the present invention, however, eventhe use of a diamine whose skeleton is a biphenyl structure cannotachieve a birefringence greater than 0.03 in some cases if the skeletonof an acid anhydride is exceedingly flexible. Therefore, it is morefavorable that both of the tetracarboxylic acid or its derivative andthe diamine have a highly linear structure in which the skeleton has norotatable bond or has only one rotatable bond.

In addition, to prevent a decrease in transparency to near-infraredlight as a result of the absorption of moisture in the air and to extendthe high-optical transparency region toward the low-wavelength side in avisible region, it is preferable that a fluorine atom be bonded to oneor both of the tetracarboxylic acid or its derivative and the diamine asthe materials. Especially when2,2'-bis(trifluoromethyl)-4,4'-diaminodiphenyl is used as the diamine,as will be presented later in the examples of the present inventionlater, it is possible to obtain a polyimide film having a large in-planebirefringence, a high optical transparency, and a low water absorption.Also, to manufacture an optical waveplate whose absorption loss tonear-infrared light containing optical communication wavelengths isreduced to a minimum possible limit, it is preferable that one or bothof the tetracarboxylic acid or its derivative and the diamine, as thematerials, be completely fluorinated except for an amino group.

A poly(amic acid) solution or film is manufactured by causing thetetracarboxylic acid or its derivative and the diamine as describedabove to react with each other. A method of manufacturing the poly(amicacid) can be the same as conventional poly(amic acid) manufacturingmethods. Generally, a dianhydride of a tetracarboxylic acid is reactedwith an equal molar quantity of a diamine in a polar organic solventsuch as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, orN,N-dimethylformamide. These materials can also be reacted in a vacuum,in a vapor phase, or at a high pressure in the absence of a solvent. Inthe present invention, both the tetracarboxylic acid or its derivativeand the diamine need not be single compounds; that is, it is possible tomix a plurality of tetracarboxylic acids or their derivatives anddiamines. In this case, the total number of moles of a plurality ofdiamines or one diamine must be equal or nearly equal to that of aplurality of tetracarboxylic acids or their derivatives or onetetracarboxylic acid or its derivative.

The resultant poly(amic acid) is then imidized to synthesize apolyimide. This synthesis can be performed by conventional polyimidesynthesizing methods including thermal imidization. In the presentinvention, however, it is also possible to obtain a mixture ofpolyimides by imidizing a plurality of poly(amic acid)s in the form of amixture, as well as imidizing a single poly(amic acid).

As a method of manufacturing a polyimide having birefringence in theplane of a film, it is effective to simultaneously or continuouslyperform uniaxial drawing and thermal imidization for a poly(amic acid)film containing a certain amount of a solvent. Specific methods that arefound to be effective by the examples of the present invention are:

a method of uniaxially drawing a poly(amic acid) film and then thermallyimidizing the film with the film be fixed in either a uniaxial orbiaxial directions by a metal frame or the like;

a method of simultaneously performing drawing and imidization byperforming thermal imidization for a poly(amic acid) film while the filmis subjected to a tensile stress in a uniaxial direction;

a method of simultaneously performing drawing and imidization by usingshrinkage of a poly(amic acid) film and evaporation of a solvent causedby imidization taking place in the process of thermal imidizationperformed for the film by fixing it in only a uniaxial direction by ametal frame or the like; and

a method of performing drawing and imidization by using the anisotropyof thermal expansion coefficient of a substrate occurring in the processof thermal imidization performed for a poly(amic acid) solution coatedon the substrate having the anisotropy of thermal expansion coefficientin its plane.

Performing the drawing simultaneously with the thermal imidization iseffective to obtain a large in-plane birefringence. However, performingthe drawing for a polyimide film which is already imidized and has noin-plane birefringence is ineffective, since the consequent in-planebirefringence is small compared to that obtained by the above method.For a polyimide film which is already imidized and yet has a retardationclose to the target value, however, performing the drawing again at ahigh temperature of 300° C. or higher is effective as a retardationadjusting method. It is also effective as a more precise retardationadjusting method to perform a thermal treatment for a polyimide film ofthe above sort at a high temperature of 300° C. or higher with no stressapplied. This method makes use of a phenomenon in which a polyimidehaving a rigid structure spontaneously orients at a high temperature toincrease the birefringence. Note that when any of these methods is to beused, it is preferable to adjust the drawing conditions or thetemperature while externally monitoring the retardation of thatpolyimide film.

One example of a method of uniaxially drawing a poly(amic acid) film ataround room temperature is a method in which a poly(amic acid) solutionis coated on a substrate, the solvent is dried to some extent, and thenthe film is peeled from the substrate and drawn. Other examples are amethod in which a poly(amic acid) solution is coated on a readilydrawable polymer (e.g., polyvinyl alcohol or polycarbonate) substrate,the solvent is dried to some extent, the poly(amic acid) film is drawntogether with the substrate, and then the film is peeled from thesubstrate; and a method in which a poly(amic acid) film peeled from asubstrate is dipped in a solvent mixture of a good solvent and a poorsolvent and drawn after the swell proceeds to a certain degree. Someother methods than the methods herein mentioned are also possible as themethod of uniaxial drawing of a poly(amic acid) at around roomtemperature or uniaxial drawing of a poly(amic acid) film at a hightemperature. That is, any method is usable in principle provided thatthe molecular chains of the poly(amic acid) or polyimide orient in theuniaxial direction. An example is a method in which a poly(amic acid)solution is coated on a substrate consisting of a heat-resistant plasticor a metal, the solvent is dried to some extent, and then the film isthermally imidized while it is drawn under a stress by bending ittogether with the substrate. Normal drawing operations using a rolldrawing machine, a tenter drawing machine, and the like are alsoconsidered to be effective.

As the substrate having an anisotropy of thermal expansion coefficientin its plane, calcite is effective as will be described later in theexamples of the present invention. Other effective examples aresingle-crystal materials such as a rock crystal, lithium niobate,lithium tantalate, and titanium oxide, and metal materials such as afiber reinforced metal (FRM) in which glass fiber or the like isembedded in the uniaxial direction, as inorganic materials; andliquid-crystal polyester, liquid crystal polyacrylate, and fiberreinformed plastic (FRP) in which glass fiber or the like is embedded inthe uniaxial direction, as organic materials. In addition, apiezoelectric material that expands or contracts in one direction uponbeing applied with a voltage and a pyroelectric material that expands orcontracts in one direction upon being heated can also be considered tobe effective as the substrate.

To obtain an optical waveplate consisting of a polyimide, it is normallyrequired to match the retardation of the polyimide to a half or quarterof the wavelength of guided light. Therefore, control of the thicknessof a film is important as well as control of the in-plane birefringence.The control of the film thickness of a polyimide is generally done byoptimizing the spin-coating conditions of a poly(amic acid) solution asa precursor of the film. A film requiring more accurate film thicknesscontrol can be formed by shaping a drawn polyimide film, with athickness slightly larger than a design value, to have a predeterminedthickness by using reactive ion etching, UV asher, or oxygen asher.

The polyimide optical waveplate according to the present invention ismanufactured for the purpose of primarily inserting it in the middle ofthe optical path of an optical waveguide or of a waveguide device.However, this polyimide optical waveplate can also be used intact as aconventional optical waveplate. It is also possible to use the polyimideoptical waveplate as an optical retardation plate by adjusting theretardation of the plate to any given value rather than a half orquarter of the wavelength of guided light. In addition, since polyimideshave a heat resistance of 300° C. or higher, it is possible to form athin film or a multilayered film of a metal, a semiconductor, or adielectric on the surface of a polyimide by sputtering or vapordeposition. Any of these films can be used as a reflecting film or afilter for cutting off light having a specific wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the dependence of excess loss on the thicknessof a waveplate when the waveplate is inserted into an optical waveguide;

FIGS. 2A and 2B are views for explaining the effect of orientation on arefractive index ellipsoid which represents the refractive indexanisotropy of a polyimide film, in which FIG. 2A illustrates arefractive index ellipsoid of a polyimide film not subjected to theorientation, and FIG. 2B illustrates an ellipsoid of a polyimide filmsubjected to the orientation, assuming that the polyimide film is formedon a substrate;

FIG. 3 is a graph showing the relationship between the weight hung froma poly(amic acid) film and the resulting in-plane birefringence;

FIG. 4 is a graph showing the relationship between the heating rateduring thermal imidization and the resulting in-plane birefringence;

FIG. 5 is a graph showing the relationship between the maximumtemperature during thermal imidization and the resulting in-planebirefringence;

FIG. 6 is a graph showing the relationship between the maximumelongation of a polyimide film during thermal imidization and theresulting in-plane birefringence;

FIG. 7 is a graph showing the wavelength dependence of both the opticaltransparency and the retardation of a PMDA/TFDB film having an in-planebirefringence;

FIG. 8 is a graph showing the wavelength dependence of both the opticaltransparency and the retardation of a PMDA/ODA film having an in-planebirefringence;

FIG. 9 is a graph showing the relationship between the spin-coatrotating speed for a poly(amic acid) solution and the retardation of apolyimide film;

FIG. 10 is a graph showing the relationship between the thermaltreatment temperature and the retardation;

FIG. 11 is a view showing a polarization convertor using a polyimidehalf waveplate according to the present invention;

FIG. 12 is a view showing a polarization-independent waveguidemulti/demultiplexer using a Mach-Zender interferometer according to thepresent invention;

FIG. 13 is a graph showing the demultiplexing characteristics of thewaveguide multi/demultiplexer shown in FIG. 12;

FIG. 14 is a view showing a polarization-independent waveguide ringresonator according to the present invention;

FIGS. 15A and 15B are graphs showing the characteristics of thewaveguide ring resonator shown in FIG. 14;

FIG. 16 is a polarization-independent waveguide multi/demultiplexerusing an arrayed-waveguide grating according to the present invention;

FIG. 17 is a graph showing the demultiplexing characteristics of thewaveguide multi/demultiplexer shown in FIG. 16;

FIG. 18 is a view showing a polarization-independent waveguidedirectional coupler according to the present invention;

FIG. 19 is a view showing a polarization-independent waveguide phasemodulator according to the present invention;

FIG. 20 is a view showing a polarization-independent waveguidepolarization beam splitter according to the present invention;

FIG. 21 is a view showing a waveguide polarization beam splitter using apolarization-independent thermo-optic phase shifter according to thepresent invention;

FIG. 22 is a view showing a polarization-independent optical circulatorusing polarization beam splitters and magnetic waveguides according tothe present invention; and

FIG. 23 is a perspective view showing a polarization convertor using apolyimide quarter waveplate and a reflecting layer according to thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in more detail below by way ofits examples. It is, however, obviously possible to obtain numerousoptical waveplates of the present invention by using various polyimidecombinations and by partially altering the drawing method. Therefore,the present invention is not limited to these examples.

The in-plane birefringence (Δn) of a polyimide film was obtained bycalculating the difference between the refractive index (n_(TE1))obtained when TE polarized light was incident in a drawing direction andthe refractive index (n_(TE2)) obtained when TE polarized light wasincident in a direction perpendicular to the drawing direction. Therefractive index was measured at a room temperature of 23° C. and awavelength of 1.55 μm by using a prism coupler (PC-2000) manufactured byMetricon Co. The film thickness (d) of a polyimide film was measuredwith the prism coupler described above, if the thickness was 20 μm orless, and was measured with a dial gauge available from Peacock Co., ifthe thickness was larger than 20 μm. A retardation (Δn×d) required toaccomplish the function as an optical waveplate can be calculated bymultiplying Δn by d obtained by the above methods. The retardation,however, can be more directly obtained by, e.g., a "Senarmont method",an "optical interference method", a "rotary analyzer method", a "phasemodulating method", or a "parallel Nicole rotation method". In eachexample, the retardation was measured by the "parallel Nicole rotationmethod" by using a laser diode with a wavelength of 1.55 μm as a lightsource and two Glan Thomson prisms as analyzers. Of the polyimides usedin the examples, a fluorinated polyimide using

2,2'-bis(trifluoromethyl)-4,4'-diaminodiphenyl as a diamine has a heatresistance higher than 300° C. and a water absorption of 0.7% or less.This has already been reported "Macromolecules" [T. Matsuura et al.,Vol. 24, p. 5,001 (1991) and T. Matsuura et al., Vol. 25, p. 3,540(1992)].

EXAMPLE 1

An N,N-dimethylacetamide solution of a poly(amic acid) synthesized frompyromellitic dianhydride (PMDA) represented by the following formula:##STR1## and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB)represented by the following formula: ##STR2## was coated on a siliconwafer 4 inches in diameter by a spin coating method. A thermal treatmentwas performed for the resultant film at 70° C. for one hour to evaporatethe solvent to such an extent that the film could be peeled. The peeledfilm was cut into a stripe 6 cm long and 3 cm wide and uniaxially drawnat room temperature by a tensile tester (Instron). Consequently, anelongation of 10% was observed. The resultant film stripe was fixed to arectangular metal frame and thermally imidized at a maximum temperatureof 350° C. for one hour. The Δn of the resultant film was found to be0.145. Assuming that the Δn of this polyimide remains unchanged, a filmthickness of 5.3 μm is necessary to use the film as a half waveplatewith a wavelength of 1.55 μm. Therefore, the spin coating conditions forthe poly(amic acid) solution were changed such that the film thicknessafter the drawing imidization became 5.3 μm, and the drawing (elongation10%) and the thermal treatment identical with those discussed above wereagain performed. Consequently, a polyimide film with Δn×d=0.775 wasobtained. Subsequently, linearly polarized light with a wavelength of1.55 μm was radiated to be incident on the resultant film such that thepolarization plane was inclined 45° from the drawing axis of the film.Consequently, it was found that the film could be used as a halfwaveplate, since the polarization plane after the transmission rotated90°. Independently, a groove 20 μm wide and 150 μm deep was cut in asilica-based buried optical waveguide at a right angle with respect tothe longitudinal direction of the waveguide. The above polyimide filmwas so cut that its drawing axis formed an angle of 45° with thewaveguide substrate. The resultant film was then inserted into thegroove, and the excess loss was measured. Consequently, the excess losswas found to be 0.3 dB.

Note that the excess loss remained unchanged even when the angle of thegroove with respect to the longitudinal direction of the waveguide wasaltered between 80° and 90°.

EXAMPLE 2

A peeled film of a poly(amic acid) formed following the same proceduresas in Example 1 was cut into a stripe 6 cm long and 3 cm wide. One endof the stripe was fixed as the upper end to a metal frame, and its otherend was pinched between two metal pieces to attach a weight of 120 g. Inthis manner, a tensile stress was applied to the film by hanging theweight from the film. The film held in this state was placed in aheating oven containing a nitrogen atmosphere and heated to a maximumtemperature of 350° C. at a heating rate of 4° C./min. Thereafter,thermal imidization was performed by holding the film at 350° C. for onehour. The Δn of the resultant film was found to be 0.037. Assuming thatthe Δn of this polyimide remains unchanged, a film thickness of 10.5 μmis required to use the film as a quarter waveplate with a wavelength of1.55 μm. Therefore, the spin coating conditions for the poly(amic acid)solution were altered such that the film thickness after the thermalimidization became 10.5 μm, and the above treatments were againperformed by changing the weight such that the same stress was appliedto the film per unit sectional area. Consequently, a polyimide film withΔn×d=0.388 was obtained.

Linearly polarized light with a wavelength of 1.55 μm was guided tobecome incident on the resultant film such that the polarization planewas inclined 45° from the drawing axis of the film. Consequently, it wasfound that the film could be used as a quarter waveplate, sincecircularly polarized light was obtained after the transmission.Following the same procedures as in Example 1, an excess loss caused byinsertion of the film into an optical waveguide was measured and foundto be 0.3 dB.

EXAMPLE 3

The following examinations were made in order to uncover the effectsthat the weight, the heating rate, and the maximum temperature had onthe Δn of the polyimide in the optical waveplate manufacturing methoddiscussed in Example 2. First, the weight was changed from 30 g to 240 gwith the heating rate and the maximum temperature fixed at 4° C./min and350° C., respectively.

As shown in FIG. 3, the Δn of the polyimide has a linear relation to theweight and can be controlled over the range of 0.017 to 0.070.Subsequently, while the weight and the maximum temperature were fixed at120 g and 350° C., respectively, the heating rate was altered from 4°C./min to 40° C./min. As shown in FIG. 4, the Δn of the polyimide has alinear relation to the heating rate and can be controlled over the rangeof 0.037 to 0.063. Lastly, the maximum temperature was changed from 350°C. to 450° C. with the weight and the heating rate fixed at 120 g and 4°C./min, respectively. As shown in FIG. 5, the Δn of the polyimide has alinear relation to the maximum temperature and can be controlled overthe range of 0.037 to 0.189. It is apparent from these results that theretardation of a polyimide film can be controlled by adjusting its Δn.As illustrated in FIG. 3, the method of changing the weight is easier torealize and can precisely control the Δn. In addition, the changeablerange of Δn is sufficient to manufacture an optical waveplate with afilm thickness of 10 to 20 μm. The method of changing the heating rateis also excellent in controllability, although the changeable range ofΔn is slightly narrow, as in FIG. 4. The method of changing the maximumtemperature is inferior in precise controllability to the other twomethods, as illustrated in FIG. 5. However, the changeable range of Δnobtained by this method is very wide, so the method is suitable for themanufacture of a waveplate with a film thickness of 10 μm or smaller. Ata maximum temperature of 450° C., for example, it is possible todecrease the thickness of a half waveplate with a wavelength of 1.30 μmto as small as 3.4 μm.

EXAMPLE 4

The following examinations were made in order to reveal the molecularstructure of the polyimide and the resultant Δn in the optical waveplatemanufacturing method discussed in Example 2. 25-μm thick films wereprepared by using, in addition to the poly(amic acid) (PMDA/TFDB)synthesized from PMDA and TFDB in Example 2, a poly(amic acid)(PMDA/ODA) synthesized from PMDA and 4,4'-diaminodiphenylether (ODA)represented by the following formula: ##STR3## a poly(amic acid)(PMDA/DMDB) synthesized from PMDA and 2,2'-dimethyl-4,4'-diaminobiphenyl(DMDB) represented by the following formula: ##STR4## a poly(amic acid)(BTDA/ODA) synthesized from 3,3',4,4'-benzophenonetetracarboxylicdianhydride (BTDA) represented by the following formula: ##STR5## andODA, a poly(amic acid) (6FDA/TFDB) synthesized from2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA)represented by the following formula: ##STR6## and TFDB, and a poly(amicacid) (PM6F/TFDB) synthesized from an equal molar mixture of PMDA and6FDA, and TFDB. Following the same procedures as in Example 3, themaximum temperature was altered between 350° C. and 450° C. with theweight and the heating rate fixed at 120 g and 4° C./min, respectively.FIG. 6 is a graph showing curves each plotting the Δn of one of theresultant polyimide films as a function of the maximum elongation ofthat film during thermal imidization. That is, FIG. 6 represents therelationship between the maximum elongation (%, plotted on the abscissa)of the polyimide film during thermal imidization and the obtainedin-plane birefringence Δn (plotted on the ordinate). As illustrated inFIG. 6, three types of the polyimides PMDA/TFDB, PMDA/ODA, and PMDA/DMDBusing PMDA as an acid anhydride can be used as the material of apolyimide optical waveplate with a film thickness of 20 μm or smaller,since they can achieve a Δn greater than 0.03 by drawing. Of thesepolyimides, in PMDA/ODA and PMDA/CMDB, the Δn tends to saturate bydrawing to a certain degree. However, no such tendency of saturation inΔn is found when PMDA/TFDB is used, even if the elongation exceeds 30%.The reason for this can be assumed that PMDA/TFDB has a linear rigidstructure and also has a trifluoromethyl group on its side chain, so theinteraction between molecular chains is relatively weak, and this allowsthe molecular chains to orient efficiently upon drawing.

Even if the diamine is the rigid TFDB, on the other hand, when the acidanhydride used is 6FDA, no birefringence greater than 0.03 can beachieved since the skeleton or main chain structure of 6FDA is veryflexible. This hindering effect that 6FDA has on Δn is large; althoughthe equal molar quantities of PMDA and 6FDA are contained in PM6F/TFDB,the increase in Δn of PM6F/TFDB with respect to the elongation is closerto that of 6FDA/TFDB than to that of PMDA/TFDB. Likewise, in the case ofBTDA/ODA in which two rotatable bonds are contained in each of the acidanhydride and the diamine, it is not possible to achieve a birefringenceexceeding 0.03. It is assumed that the Δn decreased when the elongationexceeded 20% because this polyimide was heated up to its glasstransition temperature or higher, so the orientation of molecular chainsformed by the drawing was relaxed.

FIGS. 7 and 8 are graphs showing plots of the wavelength dependence ofboth the optical transparency and the retardation measured for 15-μmthick polyimide films consisting of PMDA/TFDB and PMDA/ODA,respectively, manufactured by the above method. Referring to FIGS. 7 and8, the abscissa indicates the wavelength (μm), and the ordinateindicates the optical transparency (%) or the retardation normalizedwith 1.55 μm. Interference fringes are found in the wavelengthdependence of the optical transparency with respect to the filmthickness. The Δn at a wavelength of 1.55 μm is about 0.05 in eitherpolyimide, and the retardation is normalized with the value at 1.55 μm.It is apparent from FIGS. 7 and 8 that either polyimide has an opticaltransparency of 95% or more and a sufficient retardation in the almostentire optical communication wavelength region. In particular, thewavelength at the absorption peak, at which the optical transparencyabruptly decreases, of PMDA/TFDB containing fluorine in its molecularstructure is lower by about 0.06 μm than that of PMDA/ODA containing nofluorine. In addition, the wavelength of PMDA/TFDB, at which theretardation abruptly decreases, is also lower by about 0.1 μm than thatof PMDA/ODA. Therefore, the wavelength region of PMDA/TFDB usable as awaveplate or a retardation plate is widened accordingly.

Separately, the 15-μm thick polyimide films of PMDA/TFDB, PMDA/ODA, andPMDA/DMDB manufactured by the above method were dipped in water at roomtemperature and left to stand for ten days. Thereafter, the waterabsorption of each resultant film was measured. Consequently, the waterabsorptions of PMDA/TFDB, PMDA/ODA, and PMDA/DMDB were found to be 0.6wt %, 2.6 wt %, and 2.0 wt %, respectively. This demonstrates thatintroducing the fluorine-containing group to the polyimide molecularstructure is effective in preventing absorption of water.

EXAMPLE 5

A peeled film of a poly(amic acid) formed following the same proceduresas in Example 1 was cut into a stripe 6 cm long and 3 cm wide. This filmstripe was fixed in only a uniaxial direction to a rectangular metalframe and thermally imidized at a maximum temperature of 350° C. The Δnof the resultant film was found to be 0.053. FIG. 9 shows the filmthickness and the retardation of the polyimide film when the spincoating conditions for the poly(amic acid) solution were changed.Referring to FIG. 9, the spin-coat rotating speed (rpm) is plotted onthe abscissa, and the retardation (μm) is plotted on the ordinate. Asshown in FIG. 9, the retardation and the spin-coat rotating speed have alinear relation, so it is possible to control the retardation of thepolyimide with a high accuracy by changing the spin-coat rotating speed.FIG. 9 also reveals that since the retardation increases in proportionto the film thickness, a fixed Δn appears constantly even if the filmthickness changes. As can be seen from FIG. 9, a film thickness of 14.5μm is required to manufacture a half waveplate with a wavelength of 1.55μm. Therefore, the above treatments were again performed by setting thespin-coat rotating speed for the poly(amic acid) solution at 570 rpm.Subsequently, linearly polarized light with a wavelength of 1.55 μm wasradiated on the resultant polyimide film such that the polarizationplane was inclined 45° from the drawing axis of the film. Consequently,it was found that this film could be used as a half waveplate, since thepolarization plane after the transmission rotated 90°. Following thesame procedures as in Example 1, an excess loss caused by insertion ofthe film into an optical waveguide was measured and found to be 0.3 dB.

The half waveplates manufactured by the above method were thermallytreated at temperatures of 250° C., 300° C., 350° C., 380° C., and 400°C. each for 1 hour and cooled, and the retardation of each resultantwaveplate was measured. The measurement results are shown in FIG. 10. Asshown in FIG. 10, up to a temperature of 350° C. as the maximumtemperature in the manufacture of waveplates, the retardation remainedunchanged, and no change was found in both the film shape and theoptical transparency. Therefore, this half waveplate has a heatresistance of 350° C. However, increases in the retardation wereobserved in the waveplates thermally treated at 380° C. and 400° C. Thismeans that the molecular chains of the polyimide spontaneously orienteddue to the thermal treatment at temperatures higher than the maximumtemperature, resulting in an increased birefringence. This spontaneousorientation of polyimides at high temperatures can be used in adjustmentof the retardation, as will be described later in Example 8.

EXAMPLE 6

A poly(amic acid) solution prepared following the same procedures as inExample 1 was coated on a calcite substrate 5 cm in both length andwidth and 3 mm in thickness, in which the crystal c axis was exposed tothe plane. The resultant substrate was thermally imidized at a maximumtemperature of 350° C. The Δn of the resultant film was found to be0.031. A film thickness of 12.5 μm is required to use this polyimidefilm as a quarter waveplate with a wavelength of 1.55 μm. Therefore, theabove treatments were again performed by altering the spin coatingconditions for the poly(amic acid) solution such that the film thicknessafter the thermal imidization became 12.5 μm. Subsequently, linearlypolarized light with a wavelength of 1.55 μm was radiated on theresultant film such that the polarization plane was inclined 45° fromthe drawing axis of the film. Consequently, it was found that the filmcould be used as a quarter waveplate, since circularly polarized lightwas obtained after the transmission. Following the same procedures as inExample 1, an excess loss caused by insertion of the film into anoptical waveguide was measured and found to be 0.3 dB.

EXAMPLE 7

A poly(amic acid) solution prepared following the same procedures as inExample 1 was cast on a polycarbonate support film by using a continuousfilm formation apparatus of a solvent casting type, and passed through adrying bath at 70° C., thereby forming a film 50 cm in width and 25 μmin thickness. Thereafter, the poly(amic acid) film was peeled from thesupport film, fixed at its right and left sides in the direction ofwidth of 50 cm by a chuck, and passed through low- and high-temperaturebaths at 180° C. and 350° C., respectively. The resultant film was foundto be drawn in the direction of width of 50 cm and had a thickness of 14μm in its central portion and a Δn of 0.045. A Δn of 0.055 is necessaryto use this film as a half waveplate with a wavelength of 1.55 μm.Therefore, the polyimide film was cut into a stripe 6 cm long and 3 cmwide with the drawing direction of the film as the longitudinaldirection. One end of the film stripe was fixed as the upper end to ametal frame, and its other end was pinched between metal pieces toattach a weight of 120 g. In this manner, a tensile stress was appliedto the film by hanging the weight from the film. The film held in thisstate was placed in a heating oven containing a nitrogen atmosphere andheated at a heating rate of 4° C./min. Silica windows 5 cm in diameterare formed in the right and left sides of this heating oven, and laserlight of 1.55 μm is radiated through the polyimide film through thesewindows. A polyimide film being thermally treated can be measured byretardation measurement systems arranged on the right and the left sidesof the heating oven with the film kept placed in the oven. Theretardation began increasing when the atmospheric temperature exceeded350° C., and became 0.775 at 365° C. At that point, the heating wasstopped, and the film was naturally cooled to room temperature. When theΔn was again measured, the change in retardation was found to be 1% orless. Subsequently, linearly polarized light with a wavelength of 1.55μm was radiated on the resultant polyimide film such that thepolarization plane was inclined by 45° from the drawing axis of thefilm. Consequently, it was found that this film could be used as a halfwaveplate, since the polarization plane after the transmission rotated90°. Following the same procedures as in Example 1, an excess losscaused by insertion of the film into an optical waveguide was measuredand found to be 0.3 dB.

EXAMPLE 8

A polyimide film with a thickness of 14 μm and a Δn of 0.045manufactured following the same procedures as in Example 7 was cut intoa stripe 6 cm long and 3 cm wide with the drawing direction of the filmas the longitudinal direction. Both the ends in the direction of thedrawing axis of the film stripe were fixed to a metal frame. The filmheld in this state was placed in a heating oven containing a nitrogenatmosphere and heated at a heating rate of 4° C./min. The retardationbegan increasing when the atmospheric temperature exceeded 350° C., andbecame 0.775 at 400° C. At that point, the heating was stopped, and thefilm was naturally cooled to room temperature. When the Δn was againmeasured, the change in retardation was found to be 1% or less.Subsequently, linearly polarized light with a wavelength of 1.55 μm wasradiated on the resultant polyimide film such that the polarizationplane was inclined by 45° from the drawing axis of the film.Consequently, it was found that this film could be used as a halfwaveplate, since the polarization plane after the transmission rotated90°. Following the same procedures as in Example 1, an excess losscaused by insertion of the film into an optical waveguide was measuredand found to be 0.3 dB.

EXAMPLE 9

FIG. 11 is a view showing the ninth example of the present invention.This example is a polarization convertor constituted by one single-modewaveguide formed on a 1-mm thick silicon substrate. That is, FIG. 11 isa schematic view showing a polarization convertor using a polyimide halfwaveplate according to the present invention. Referring to FIG. 11,reference numeral 1 denotes an input waveguide; 2, an output waveguide;3, a polyimide half waveplate; 4, a groove; and 5, a silicon substrate.

This waveguide is a silica-based waveguide formed by flame hydrolysisdeposition and reactive ion etching. The waveguide has a sectionalstructure in which a core with dimensions of 7 μm×7 μm is buried insubstantially the center of a 60-μm thick cladding layer deposited onthe silicon substrate. The specific refractive index difference betweenthe cladding and the core is 0.75%. A groove 20 μm wide and 150 μm deepis formed in the middle of the optical path so as to form an angle of86° with the optical waveguide. This angle formed between the groove andthe optical waveguide is preferably an angle slightly shifted from 90°in order to reduce light reflected by the surface of the waveplate. If,however, the angle is largely shifted from 90°, the retardation of thewaveplate also is shifted from the design value. Therefore, an anglefrom 80° to 86° is normally used. In addition, a similar effect can beobtained when the groove is formed to be not perpendicular to butslightly inclined from the substrate. This is obvious from the aboveexplanation. This groove can be formed by either chemical processing,such as etching, or mechanical processing using, e.g., a dicing saw. Inthis example, the groove was formed by a dicing saw using a blade 15 μmin thickness. The 14.5-μm polyimide half waveplate manufactured inExample 5 and so cut that its optical principal axis formed an angle of45° with the substrate was inserted into the groove.

A polarization-maintaining single-mode optical fiber was connected tothe input waveguide 1 of this polarization convertor, and polarizedlight (horizontal polarization) having an electric field parallel to thewaveguide substrate 5 was input. Consequently, polarized light (verticalpolarization) having an electric field perpendicular to the waveguidesubstrate 5 emerged from the output waveguide 2. Likewise, horizontalpolarization emerged from the output waveguide 2 when verticalpolarization was input. A polarization mode conversion ratio indicativeof the efficiency at which horizontal polarization was converted intovertical polarization or vice versa was measured and found to be 30 dB.An excess loss caused by insertion of the polyimide half waveplate 3into the groove 4 was found to be 0.3 dB.

EXAMPLE 10

FIG. 12 is a view showing the tenth example of the present invention. Inthis example, the polarization convertor of the present invention wasapplied to a waveguide multi/demultiplexer using a Mach-Zenderinterferometer constituted by two single-mode optical waveguides. Thatis, FIG. 12 is a schematic view showing a polarization-independentwaveguide multi/demultiplexer using the Mach-Zender interferometeraccording to the present invention. Referring to FIG. 12, referencenumerals 3 to 5 denote the same parts as in FIG. 11; 6, a first inputwaveguide; 7, a second input waveguide; 8, a first output waveguide; 9,a second output waveguide; 10, a first directional coupler; 11, a firstoptical path; 12, a second optical path; and 13, a second directionalcoupler. The too waveguides constitute the first input waveguide 6, thesecond input waveguide 7, the first directional coupler 10, the seconddirectional coupler 13, the first optical path 11, the second opticalpath 12, the first output waveguide 8, and the second output waveguide9. The coupling ratios of both the first and second directional couplers10 and 13 are 50%. The length of the first optical path 11 is differentby ΔL from that of the second optical path 12. A groove 4 is formed inthe middle of the first and second optical paths 11 and 12, and apolyimide half waveplate 3 is inserted into the groove. The dimensions,the manufacturing conditions, and the propagation characteristics of thewaveguides, the angle formed between the optical principal axis of thewaveplate and the waveguide substrate, the shapes of the groove and thewaveplate, the angle formed between the groove and the waveguides, andthe characteristics of the waveplate used in this example are the sameas those in Example 9. The polyimide optical waveplate 3 acts as apolarization convertor to convert horizontal polarization of guidedlight propagating through the first and second optical paths 11 and 12into vertical polarization, and vertical polarization into horizontalone. An optical fiber is connected to the first input waveguide 6. Notethat connecting an optical fiber to the second input waveguide 7 has noinfluence on the operation of the waveguide multi/demultiplexer of thisexample, although the first and second outputs change places with eachother in the following description. The guided light from the firstinput waveguide 6 is equally divided in power by the first directionalcoupler 10. The divided light components independently propagate throughthe first and second optical paths 11 and 12 and are again coupledtogether by the second coupler 13. The resultant light is extracted fromthe first and second output waveguides 8 and 9.

Assume that there is no polarization convertor using the polyimideoptical waveplate. In this case, since the silica-based waveguidesformed on the silicon substrate have birefringence, the refractive indexto horizontal polarization differs from that to vertical polarization.Consequently, the optical path length difference between the first andsecond optical paths when horizontal polarization is incident isdifferent from that when vertical polarization is incident. This givesthe multi/demultiplexer a polarization dependence. In this case, theoptical path length is a value calculated by multiplying the distancethe light is guided by the refractive index, and is proportional to thephase delay caused by the propagation of light. In contrast, when thepolarization convertor is arranged in the middle of the first and secondoptical paths, as in FIG. 12, the optical path length difference forhorizontal polarization is equal to that for vertical polarization. Thisis so because light incident by horizontal polarization is subjected tothe refractive index as that of horizontal polarization in the firsthalf of the optical path but to the refractive index as that of verticalpolarization in the last half, so the total optical path length is theproduct of the mean refractive index and the physical length. Similarly,the optical path length for light incident by vertical polarization isalso the product of the mean of the refractive index for horizontalpolarization and that for vertical polarization and the physical length.Consequently, the multi/demultiplexer of this example becomespolarization-independent.

FIG. 13 is a graph showing the demultiplexing characteristics of thewaveguide multi/demultiplexer illustrated in FIG. 12. Referring to FIG.13, the abscissa indicates the signal light wavelength, and the ordinateindicates the intensity of transmitted light. The curves in FIG. 13represent the multi/demultiplexing characteristic when the polarizationconvertor using the polyimide optical waveplate is present (a solidline), that when a polarization convertor using a conventionalrock-crystal optical waveplate is present (an alternate long and shortdashed line), and that when there is no polarization convertor (a dottedline). The results of FIG. 13 were obtained by inputting equal lightquantities of horizontal polarization and vertical polarization as inputlight from the first input waveguide, and measuring the output from thefirst output waveguide. The horizontal and vertical polarizations havetheir respective transmission spectra represented by sinusoidal waves.In the absence of the polarization convertor, however, the transmissionspectrum of the horizontal polarization differs from that of thevertical polarization. Consequently, the total transmission spectrumrepresented by the sum of these transmission spectra has a lowextinction ratio. The extinction ratio is the ratio of the output at awavelength at which light is output most intensely to the output at awavelength at which light is output most weakly. When there is thepolarization convertor using the rock-crystal optical waveplate, theextinction ratio rises because the transmission spectra of thehorizontal and vertical polarizations agree with each other. Since,however, the rock-crystal waveplate is thick, the excess loss becomes aslarge as 4 dB. On the other hand, when the polarization convertor usingthe polyimide optical waveplate is present, the polarization dependenceis eliminated, and this results in a high extinction ratio and a verysmall excess loss of 0.3 dB.

EXAMPLE 11

FIG. 14 shows the 11th example of the present invention, in which thepolarization convertor of the present invention was applied to awaveguide ring resonator. That is, FIG. 14 is a schematic view showing apolarization-independent waveguide ring resonator according to thepresent invention. Referring to FIG. 14, reference numerals 1 to 5denote the same parts as in FIG. 11; 10 and 13, the same parts as inFIG. 12; 14, an input fiber; 15, a ring waveguide; and 16, an outputfiber. On a silicon substrate 5, an input waveguide 1, the ringwaveguide 15, and an output waveguide 2 are arranged. The inputwaveguide 1 and the ring waveguide 15 are coupled by a first directionalcoupler 10, and the output waveguide 2 and the ring waveguide 15 arecoupled by a second directional coupler 13. A groove 4 is formed at twopositions (intermediate positions viewed from the directional couplers10 and 13) of the ring waveguide 15. Polyimide half waveplates 3 areinserted into this groove. The dimensions, the manufacturing conditions,and the propagation characteristics of the waveguides, the angle formedbetween the optical principal axis of the waveplate and the waveguidesubstrate, the shapes of the groove and the waveplate, the angle formedbetween the groove and the waveguides, and the characteristics of thewaveplate used in this example are the same as those in Example 9. Theprinciple of this example is also the same as in Example 10. In theabsence of the optical waveplates 3, there is a difference in opticalpath length upon one propagation along the ring resonator betweenhorizontal polarization and vertical polarization due to thebirefringence of the waveguides. To compensate for this difference, thehalf waveplates 3 were inserted to function as a polarization convertor,thereby eliminating the polarization dependence. FIG. 15A shows thetransmission spectrum of the ring resonator when the polyimide opticalwaveplates of this example were inserted. For comparison, FIG. 15B showsthe transmission spectrum of the ring resonator when there was nooptical waveplate. In FIGS. 15A and 15B, the wavelength is plotted onthe abscissa, and the intensity of transmitted light (given unit) isplotted on the ordinate. The use of the polarization convertorincorporating the polyimide optical waveplates made it possible toobtain a loss of one tenth or less of that when the polarizationconvertor using the rock-crystal waveplates was used.

The polyimide optical waveplates were inserted at two positions in thisexample, but the present invention is not limited to this example. Thatis, it is obvious that similar effects can be obtained if only an evennumber of waveplates are inserted. If the number of waveplates is an oddnumber, such as 1 or 3, the cavity length is doubled while the effect ofeliminating the polarization dependence remains the same. This meansthat miniaturization is possible because the length of the ringwaveguide can be halved.

EXAMPLE 12

FIG. 16 shows the 12th example of the present invention, in which thepolarization convertor of the present invention was applied to amulti/demultiplexer using an arrayed-waveguide grating. That is, FIG. 16is a schematic view showing a polarization-independent waveguidemulti/demultiplexer using the arrayed-waveguide grating according to thepresent invention. Referring to FIG. 16, reference numerals 1 to 5denote the same parts as in FIG. 11; 17, a first slab waveguide; 18, asecond slab waveguide; 19, channel waveguides; and 20, an arrayedwaveguide.

On a silicon substrate 5, an input waveguide 1, the first slab waveguide17, the arrayed waveguide 20, the second slab waveguide 18, and aplurality of output waveguides 2 are connected in this order. The twoslab waveguides 17 and 18 are connected to a plurality of the channelwaveguides 19 formed into sectors having the ends of the input waveguide1 and the output waveguides, respectively, as the centers of curvature.The arrayed waveguide 20 is constituted by a plurality of channelwaveguides whose lengths differ from one another by ΔL. One commongroove 4 is formed in a central portion of these channel waveguides 19,and a polyimide half waveplate 3 is inserted into the groove 4. Thedimensions, the manufacturing conditions, and the propagationcharacteristics of the waveguides, the angle formed between the opticalprincipal axis of the waveplate and the waveguide substrate, the shapesof the groove and the waveplate, the angle formed between the groove andthe waveguides, and the characteristics of the waveplate used in thisexample are the same as those in Example 9. The half waveplate 3 must bearranged at the midpoint of each of the channel waveguides 19. In thisexample, therefore, the arrayed waveguide 20 is designed symmetricallysuch that the midpoints of the channel waveguides 19 are arranged inline. Consequently, the groove 4 is formed as one continuous straightline. In this case, the polyimide half waveplate 3 reed only be a singlewaveplate having a length by which the waveplate traverses all thechannel waveguides 19 constituting the arrayed waveguide 20. The arrayedwaveguide may not be symmetrical depending on the design. In such acase, the same number of half waveplates as the number of the channelwaveguides 19 must be inserted, since the groove is not formed in line.This is unfavorable because the amount of work increases.

The effect of eliminating the polarization dependence when thepolarization convertor of the present invention is applied to thearrayed-waveguide grating is identical with that of the Mach-Zenderinterferometer in Example 10. FIG. 17 is a graph showing thedemultiplexing characteristics of the waveguide multi/demultiplexerillustrated in FIG. 16. In FIG. 17, the signal light wavelength isplotted on the abscissa, and the loss is plotted on the ordinate. Thecurves in FIG. 17 represent the multi/demultiplexing characteristic whenthe polarization convertor using the polyimide optical waveplate ispresent (a solid line), that when a polarization convertor using aconventional rock-crystal optical waveplate is present (an alternatelong and short dashed line), and that when there is no polarizationconvertor (a dotted line). When the polarization convertor incorporatingthe polyimide optical waveplate is used, the polarization dependence iseliminated, and the loss largely decreases to 0.3 dB.

EXAMPLE 13

FIG. 18 shows the 13th example of the present invention, in which thepolarization convertor of the present invention was applied to adirectional coupler. That is, FIG. 18 is a schematic view showing apolarization-independent waveguide directional coupler according to thepresent invention. Referring to FIG. 18, reference numerals 3 to 9denote the same parts as in FIG. 12; and 21, a directional coupler.

On a silicon substrate 5, a first input waveguide 6, a second inputwaveguide 7, the directional coupler 21, a first output waveguide 8, anda second output waveguide 9 are formed. A groove 4 is formed in themiddle of the directional coupler 21, and a polyimide half waveplate 3is inserted into the groove 4. The dimensions, the manufacturingconditions, and the propagation characteristics of the waveguides, theangle formed between the optical principal axis of the waveplate and thewaveguide substrate, the shapes of the groove and the waveplate, theangle formed between the groove and the waveguides, and thecharacteristics of the waveplate used in this example are the same asthose in Example 9. The length L of the directional coupler 21 isone-half of the unity coupling length. This device is so designed as tooperate as a 3 dB coupler (coupling ratio 1:1). However, the presentinvention is not limited to this example but applicable to directionalcouplers having various coupling ratios.

Assume the effective refractive indices of two propagation modes (aneven mode and an odd mode) of the direction coupler are n_(e) and n_(o),respectively, and horizontal and vertical polarizations are givensubscripts (TE) and (TM), respectively. The even and odd modes areexcited at the left end of the directional coupler by light propagatingthrough the first input waveguide. Since the horizontal and verticalpolarizations are switched during the propagation, the differences inoptical path length between the even and odd modes are given as follows:For input of horizontal polarization,

    (n.sub.e(TE) L/2+n.sub.e(TM) L/2)-(n.sub.o(TE) L/2+n.sub.o(TM) L2)(1)

For input of vertical polarization,

    (n.sub.e(TM) L/2+n.sub.e(TE) L/2)-(n.sub.o(TM) L/2+n.sub.o(TE) L/2)(2)

That is, the two values are in agreement. Therefore, the coupling ratioof the direction coupler has no polarization dependence. The length L ofthe directional coupler 21 is so set that the value of Equation (1) and(2) is a quarter of the wavelength. Therefore, light components equallydistributed (1:1) are extracted from the first and second outputwaveguides 8 and 9. The present invention is of course not limited tothe directional coupler having the coupling ratio of 1:1 but isapplicable to those having various coupling ratios. When thepolarization convertor incorporating the polyimide optical waveplate 3was used as the directional coupler 21 of this example, no polarizationdependence of the coupling ratio was found, and the excess loss was 0.3dB.

EXAMPLE 14

FIG. 19 shows the 14th example of the present invention, in which thepolarization convertor of the present invention was applied to a phasemodulator. That is, FIG. 19 is a schematic view showing apolarization-independent waveguide phase modulator according to thepresent invention. Referring to FIG. 19, reference numerals 3 and 4denote the same parts as in FIG. 11; 22, positive electrodes; 23,negative electrodes; 24, an LiNbO₃ substrate; and 25, a Ti in-diffusedsubstrate.

A titanium (Ti) film was deposited on the mirror-polished lithiumniobate (LiNbO₃) substrate 24 and patterned. Thereafter, Ti wasthermally diffused in a high-temperature atmosphere at about 1,000° C.to form the optical waveguide 25. In addition, the gold (Au) electrodes22 and 23 were formed near the waveguide 25, thereby manufacturing aphase modulator. When a voltage is applied across the positive andnegative electrodes in FIG. 19, the refractive index of the waveguide 25changes due to the electrooptic effect. However, since the change in therefractive index brought about by the electrooptic effect has apolarization dependence, the change in phase of light also has adifference between horizontal polarization and vertical polarization.Therefore, a groove 4 was formed at the center of the phase modulator ina direction perpendicular to the waveguide 25, and a polyimide halfwaveplate 3 was inserted into this groove. In this case, by insertingthe polyimide half waveplate 3 as a polarization convertor such that itsoptical principal axis formed an angle of 45° with the waveguidesubstrate 25, a polarization-independent phase modulator was realized.The dimensions, the manufacturing conditions, and the propagationcharacteristics of the waveguides, the angle formed between the opticalprincipal axis of the waveplate and the waveguide substrate, the shapesof the groove and the waveplate, the angle formed between the groove andthe waveguides, and the characteristics of the waveplate used in thisexample are the same as those in Example 9. The excess loss was found tobe 2.0 dB when the polarization convertor incorporating the polyimideoptical waveplate 3 was used. The LiNbO₃ substrate has a largerbrittleness than that of silica, so it is difficult to accurately formgrooves in this substrate. Therefore, it is estimated that this largeexcess loss was caused by an unsatisfactory processing accuracy of thegroove.

EXAMPLE 15

FIG. 20 shows the 15th example of the present invention, in which thepolarization convertor of the present invention was applied to apolarization beam splitter. That is, FIG. 20 is a schematic view showinga polarization-independent waveguide polarization beam splitteraccording to the present invention. Referring to FIG. 20, referencenumerals 4 to 13 denote the same parts as in FIG. 12; and 36, polyimidequarter waveplates.

This waveguide device is identical with that discussed in Example 10except that the optical path length difference between first and secondoptical paths 11 and 12 is a quarter wavelength (λ/4), the polyimideoptical waveplates 36 inserted into the optical paths are quarterwaveplates rather than half waveplates, and the angle formed between theoptical principal axis of each of the optical waveplates 36 and asubstrate is not 45°. The optical principal axis of the opticalwaveplate 36 inserted into the first optical path 11 is perpendicular toa waveguide substrate 5. Therefore, although there is no couplingbetween polarization modes, vertical polarization has an optical pathlength longer by a quarter wavelength than that for horizontalpolarization in the first optical path 11. On the other hand, theoptical principal axis of the quarter waveplate 36 inserted into thesecond optical path 12 is parallel to the waveguide substrate 5.Therefore, horizontal polarization has an optical path length longer bya quarter wavelength than that for vertical polarization in the secondoptical path 12. In addition, the second optical path 12 is formed to belonger by a quarter wavelength than the first optical path 11 by theoriginal circuit design. Consequently, the optical path lengths of theindividual modes are as follows:

    ______________________________________                                        Vertical polarization                                                                              α + /4                                             in 1st optical path                                                           Horizontal polarization                                                                            α                                                  in 1st optical path                                                           Vertical polarization                                                                              α + /4                                             in 2nd optical path                                                           Horizontal polarization                                                                            α + /4 + /4                                        in 2nd optical path                                                           ______________________________________                                    

That is, there is no optical path length difference between the two armwaveguides with respect to vertical polarization. Therefore, input lightfrom a first input waveguide 6 is output from a second output waveguide9 as a cross port.

Since, on the other hand, an optical path length difference of a halfwavelength is present between the arm waveguides with respect tohorizontal polarization, input light from the first input waveguide 6 isoutput from a first output waveguide 8 as a through port.

More specifically, this circuit functions as a polarization beamsplitter.

The dimensions, the manufacturing conditions, and the propagationcharacteristics of the waveguides, the angle formed between the opticalprincipal axis of the waveplate and the waveguide substrate, the shapesof the groove and the waveplate, the angle formed between the groove andthe waveguides, and the characteristics of the waveplate used in thisexample are the same as those in Example 9. The polyimide quarterwaveplates 36 used were those manufactured in Example 2. When verticalpolarization was input from the first input waveguide 6, the light wasoutput from the second output waveguide 9 as a cross port. Whenhorizontal polarization was input from the first input waveguide 6, thelight was output from the first output waveguide 8 as a through port.The excess loss was found to be 0.3 dB when the polarization convertorincorporating the polyimide optical waveplates 36 was used.

In this example, the method using two quarter waveplates has beenexplained. However, as illustrated in FIG. 21, it is also possible torealize a polarization beam splitter by inserting a half waveplate 3into a groove 4, which is formed in a first optical path 11, such thatthe optical principal axis of the waveplate is parallel or perpendicularto a waveguide substrate 5, and by arranging a phase controller, such asa thermo-optic phase shifter 26, in a second optical path 12. Thethermo-optic phase shifter 26 shown in FIG. 21 is manufactured byforming a thin-film heater on the surface of a waveguide. Thethermo-optic phase shifter 26 controls the waveguide temperature byheating this thin-film heater, thereby controlling the phase of light byusing the thermo-optic effect.

EXAMPLE 16

FIG. 22 shows the 16th example of the present invention. That is, FIG.22 is a schematic view showing a polarization-independent opticalcirculator using polarization beam splitters and magnetic waveguidesaccording to the present invention. Referring to FIG. 22, referencenumerals 3, 4, and 6 to 9 denote the same parts as in FIG. 12; 27, afirst polarization beam splitter consisting of nonmagnetic waveguides;28, a first output waveguide of the first polarization beam splitter;29, a second output waveguide of the first polarization beam splitter;30, magnetic waveguides; 31, a nonreciprocal device consisting of themagnetic waveguides; 32, a second polarization beam splitter consistingof nonmagnetic waveguides; 33, a first input waveguide of the secondpolarization beam splitter; and 34, a second input waveguide of thesecond polarization beam splitter. This waveguide device is constitutedby the polarization beam splitters discussed in Example 15, the magneticwaveguides, and the polyimide optical waveplate of the presentinvention.

The operating principle of the device when light is input from the firstinput waveguide 6 will be described first. The input light from thefirst input waveguide 6 is split by the first polarization beamsplitter. Consequently, the vertical polarization of the input light istransmitted to the second output waveguide 29 of the first polarizationbeam splitter as a cross port, and the horizontal polarization of theinput light is transmitted to the first output waveguide 28 of the firstpolarization beam splitter as a through port. The device is so designedthat these light components are subjected to Faraday rotation in themagnetic waveguides 30 to rotate their polarization planes 45°. Inaddition, the polarization planes of the transmitted light componentsare further rotated 45° since the polyimide half waveplate is arrangedsuch that its optical principal axis is inclined 22.5° or 67.5° from awaveguide substrate 5. As a result, the output horizontal polarizationfrom the first output waveguide 28 of the first polarization beamsplitter is converted into vertical polarization and input to the firstinput waveguide 33 of the second polarization beam splitter. On theother hand, the output vertical polarization from the second outputwaveguide 28 of the first polarization beam splitter is converted intohorizontal polarization and input to the second input waveguide 34 ofthe second polarization beam splitter. Thereafter, since the verticaland horizontal polarizations are transmitted to the cross and throughports, respectively, by the second polarization beam splitter, the twopolarizations are multiplexed and output from the second outputwaveguide 9. Consequently, the input light from the first inputwaveguide 6 is output from the second output waveguide 9, and the inputlight from the second input waveguide 7 is output from the first outputwaveguide 8, independent of their respective polarized states.

Consider next the case in which the input ports are switched, i.e.,light is input from the second output waveguide 9. In this case, thevertical polarization is transmitted to the first input waveguide 33 ofthe second polarization beam splitter as the cross port by the secondpolarization beam splitter. The horizontal polarization, on the otherhand, is transmitted to the second input waveguide 34 of the secondpolarization beam splitter as the through port. Thereafter, thepolarization planes of these light components are rotated 45° by thepolyimide half waveplate 3.

The operation to this point is a reversible operation because of theprinciple of a reciprocal device. Since, however, the magnetic waveguide31 is a nonreciprocal device, the rotating direction of the polarizationplane when light is transmitted from the right to the left in FIG. 22 isopposite to that when light is transmitted from the left to the right.For this reason, the input vertical polarization from the first inputwaveguide 33 of the second polarization beam splitter is transmittedintact to the first output waveguide 28 of the first polarization beamsplitter, and the input horizontal polarization from the second inputwaveguide 34 of the second polarization beam splitter is transmittedintact to the second output waveguide 29 of the first polarization beamsplitter. These light components are multiplexed by the firstpolarization beam splitter and output from the second input waveguide 7.Likewise, input light from the first output waveguide 8 is output fromthe first input waveguide 6, independent of the polarized state of thatlight. That is, this waveguide device functions as apolarization-independent circulator. Note that this device can alsofunction as a polarization-independent waveguide isolator by inputtinglight from the first input waveguide 6 and extracting it from the secondoutput waveguide 9.

The dimensions, the manufacturing conditions, and the propagationcharacteristics of the waveguides, the shapes of the grooves and thewaveplate, and the characteristics of the waveplate are the same asthose in Example 9. In accordance with the design of the waveguidecircuit, input light from the first input waveguide 6 was output fromthe second output waveguide 9, and input light from the second inputwaveguide 7 was output from the first output waveguide 8, independent oftheir respective polarized states. Similarly, input light from the firstoutput waveguide 8 was output from the first input waveguide 6, andinput light from the second output waveguide 9 was output from thesecond input waveguide 7, independent of their respective polarizedstates. The total excess loss was found to be 0.9 dB when thepolarization convertor incorporating the polyimide optical plates 36 and33 was used.

EXAMPLE 17

FIG. 23 is a view for explaining the 17th example of the presentinvention. That is, FIG. 23 is a schematic view showing a polarizationconvertor using the polyimide quarter waveplate according to the presentinvention and a reflecting layer. Referring to FIG. 23, referencenumerals 1, 2, and 5 denote the same parts as in FIG. 11; 35, adielectric multilayered interference filter; and 36, a polyimide quarterwaveplate.

The principle of this waveguide device is identical with that of thepolarization convertor of Example 9 except that polarization conversionis performed by using the polyimide quarter waveplate 36 and thereflecting film 35. The polyimide quarter waveplate 36 arranged at theend face of a waveguide is bonded such that the optical principal axisforms an angle of 45° with a waveguide substrate 5. The reflecting coat35 for reflecting guided light is formed on the surface of the opticalwaveplate 36 away from the surface in contact with the waveguide. Inthis example, the reflecting coat is formed by using the dielectricmultilayered interference film. However, it is also possible to use ametal reflecting film as the reflecting coat. The dimensions, themanufacturing conditions, and the propagation characteristics of thewaveguide used in this example are the same as those in Example 9. Inputlight from an input waveguide 1 is transmitted through the polyimidequarter waveplate 36 and reflected by the dielectric multilayeredinterference film 35. The reflected light is again transmitted throughthe quarter waveplate 36 and input to an output waveguide 2.Consequently, since the light is transmitted through the quarterwaveplate 36 twice, the same effect as when light is transmitted througha half waveplate can be obtained.

A polarization-maintaining single-mode optical fiber was connected tothe input waveguide 1 of this polarization convertor, and polarizedlight (horizontal polarization) having an electric field parallel to thewaveguide substrate 5 was input. Consequently, polarized light (verticalpolarization) having an electric field perpendicular to the waveguidesubstrate 5 emerged from the output waveguide 2. Likewise, horizontalpolarization emerged from the output waveguide 2 when verticalpolarization was input. A polarization mode conversion ratio indicativeof the efficiency at which horizontal polarization was converted intovertical polarization or vice versa was measured and found to be 30 dB.

The advantage of this example is that no groove for receiving theoptical waveplate need be formed in a waveguide circuit. As discussed inExample 14, a substrate consisting of, e.g., LiNbO₃ has a largebrittleness, so it is difficult to accurately form grooves in thissubstrate. It is therefore considered to be effective to apply themethod of this example to a waveguide device formed on a substrate ofthis type.

In this example, the input and output waveguides were separately formed.However, it is also possible to use a single waveguide as the input andoutput waveguides.

Comparative Example 1

A groove 100 μm wide and 100 μm deep was cut in a silica-based buriedoptical waveguide at a right angle with respect to the direction of thewaveguide. A half waveplate (thickness 91 μm) consisting of a rockcrystal and having a wavelength of 1.55 μm was cut such that its opticalprincipal axis formed an angle of 45° with a waveguide substrate. Theresultant waveplate was inserted into the groove, and the excess losswas measured. Consequently, the excess loss was found to be 4 dB.

Comparative Example 2

The poly(amic acid) solution prepared in Example 1 was coated on asilicon wafer 4 inches in diameter by a spin coating method andthermally imidized at a maximum temperature of 350° C. The resultantfilm was peeled from the substrate and cut into a stripe. The obtainedfilm stripe was uniaxially drawn at room temperature by using a tensiletester (Instron). Consequently, the film was broken when elongated byabout 1%. The resultant film was found to have a film thickness of 10.1μm and a Δn of 0.0008. A film thickness of about 1 mm is required to usethis polyimide film as a half waveplate with a wavelength of 1.55 μm,and the expected insertion loss is assumed to be 40 dB or larger. It wasconsequently found that this polyimide film could not be used as anoptical waveplate.

According to the present invention, a polyimide film with a filmthickness of 20 μm or smaller is used. Therefore, in place of opticalwaveplates using conventional inorganic single-crystal materials, it ispossible to provide an optical waveplate which is easy to manufactureand has a high flexibility. This optical waveplate causes littleinsertion loss, since the film thickness is smaller than that of anoptical waveplate using a rock crystal, and also has a high heatresistance of 300° C. or higher. This makes it possible primarily toimprove the performance of waveguide devices, reduce the manufacturingcost, and increase the efficiency in manufacturing processes. Inaddition, by inserting the optical waveplate into various lightwavecircuits as discussed in the examples, it is also possible to improvethe function and the performance of the devices, and to manufacturenovel waveguide devices.

What is claimed is:
 1. A waveguide device constituted by an opticalwaveguide formed on a substrate, wherein a groove is so formed as toform a predetermined angle, which is close to a right angle, with saidwaveguide and to cross said waveguide, and an optical waveplateconsisting of a polyimide film with a film thickness of not more than 20μm is inserted into said groove.
 2. A polarization convertor, wherein awaveguide device comprises a single optical waveguide, and a halfwaveplate consisting of a polyimide film with a film thickness of notmore than 20 μm and having a product of an in-plane anisotropy ofrefractive index and a thickness set to one-half of a wavelength oflight propagating through said waveguide is inserted into a grooveformed in a waveguide substrate so as to cross said waveguide substrateand to make a certain angle with said waveguide.
 3. A convertoraccording to claim 2, wherein the certain angle is defined between 80°and 90°.
 4. A waveguide device, wherein said waveguide device is aMach-Zender interferometer arranged on a waveguide substrate andcomprising two input waveguides, a first directional coupler connectedto said input waveguides, two output waveguides, a second directionalcoupler connected to said output waveguides, and first and secondwaveguides connecting said first and second directional couplers, and apolarization convertor consisting of a polyimide film with a filmthickness of not more than 20 μm is arranged in the middle of an opticalpath of each of said first and second waveguides so as to form a certainangle with said waveguide substrate.
 5. A device according to claim 4,wherein the certain angle is defined between 80° and 90°.
 6. A waveguidedevice, wherein said waveguide device is a ring resonator arranged on awaveguide substrate and comprising a plurality of input/outputwaveguides, and a ring waveguide coupled to said input/outputwaveguides, and at least one polarization convertor consisting of apolyimide film with a film thickness of not more than 20 μm is arrangedon said ring waveguide so as to form a certain angle with said waveguidesubstrate, in order that an equal optical path length of said ringwaveguide is obtained for input horizontal polarization and verticalpolarization.
 7. A device according to claim 6, wherein the certainangle is defined between 80° and 90°.
 8. A waveguide device, whereinsaid waveguide device is a directional coupler formed on a waveguidesubstrate, and a polarization convertor consisting of a polyimide filmwith a film thickness of not more than 20 μm is arranged in the middleof said directional coupler so as to form a certain angle with saidwaveguide substrate.
 9. A device according to claim 8, wherein thecertain angle is defined between 80° and 90°.
 10. A waveguide device,wherein said waveguide device is a phase modulator comprising awaveguide having an electrooptic effect, and a pair of electrodesarranged near said waveguide, and at least one polarization convertorconsisting of a polyimide film with a film thickness of not more than 20μm is arranged in the middle of said waveguide so as to form a certainangle with a waveguide substrate.
 11. A device according to claim 10,wherein the certain angle is defined between 80° and 90°.
 12. Awaveguide device, wherein said waveguide device is a polarization beamsplitter arranged on a waveguide substrate and comprising two inputwaveguides, a first directional coupler connected to said inputwaveguides, two output waveguides, a second directional couplerconnected to said output waveguides, and first and second waveguidesconnecting said first and second directional couplers, said first andsecond waveguides have an optical path length difference which is aquarter of a wavelength of guided light, and waveplates each consistingof a polyimide film with a film thickness of not more than 20 μm arearranged in optical paths of said first and second waveguides so as toform certain angles with said waveguides.
 13. A device according toclaim 12, wherein each of said waveplates is a quarter waveplate havinga product of an in-plane anisotropy of refractive index and a thicknessset to a quarter of a wavelength of light propagating through saidwaveguides, said quarter waveplates being inserted into said waveguidesubstrate such that optical principal axes of said quarter waveplatesare, respectively, perpendicular and parallel to said waveguidesubstrate.
 14. A waveguide device, wherein said waveguide device is apolarization beam splitter arranged on a waveguide substrate andcomprising two input waveguides, a first directional coupler connectedto said input waveguides, two output waveguides, a second directionalcoupler connected to said output waveguides, and first and secondwaveguides connecting said first and second directional couplers, saidfirst and second waveguides have an optical path length difference whichis a quarter of a wavelength of guided light, and a waveplate consistingof a polyimide film with a film thickness of not more than 20 μm isarranged in an optical path of said first waveguide so as to form acertain angle with said first waveguide.
 15. A device according to claim14, wherein said waveplate is a half waveplate, said half waveplatebeing inserted into said waveguide substrate such that an opticalprincipal axis of said half waveplate is perpendicular to said waveguidesubstrate, and a thermo-optic phase shifter is arranged in an opticalpath of said second waveguide.
 16. A device according to claim 14,wherein said waveplate is a half waveplate, said half waveplate beinginserted into said waveguide substrate such that an optical principalaxis of said half waveplate is parallel to said waveguide substrate, anda thermo-optic phase shifter is arranged in an optical path of saidsecond waveguide.
 17. A waveguide device comprising a first 2×2polarization beam splitter constituted by nonmagnetic waveguides, twononreciprocal elements constituted by magnetic waveguides, and a second2×2 polarization beam splitter, wherein a waveplate consisting of apolyimide film with a film thickness of not more than 20 μm is arrangedbetween said two nonreciprocal elements and said second 2×2 polarizationbeam splitter so as to form a certain angle with a waveguide substrate.18. A device according to claim 14, wherein said waveplate is a halfwaveplate, said half waveplate being inserted into said waveguidesubstrate such that an optical principal axis of said half waveplateforms an angle of 22.5° with said waveguide substrate.
 19. A deviceaccording to claim 14, wherein said waveplate is a half waveplate, saidhalf waveplate being inserted into said waveguide substrate such that anoptical principal axis of said half waveplate forms an angle of 67.5°with said waveguide substrate.
 20. A waveguide device comprising anoptical waveguide formed on a substrate, an optical waveplate consistingof a polyimide film with a film thickness of not more than 20 μm andformed in tight contact with an end face of said optical waveguide so asto be perpendicular to a longitudinal direction of said opticalwaveguide, and a reflecting coat, formed on a surface of said opticalwaveplate away from the end face of said waveguide, for reflectingguided light.
 21. A waveguide device comprising an optical waveguideformed on a substrate, an optical waveplate consisting of a polyimidefilm with a film thickness of not more than 20 μm and formed in tightcontact with an end face of said optical waveguide so as to be inclinedfrom a longitudinal direction of said optical waveguide, and areflecting coat, formed on a surface of said optical waveplate away fromthe end face of said waveguide, for reflecting guided light.
 22. Awaveguide device comprising a polarization convertor including at leastone optical waveguide formed on a waveguide substrate, and an opticalwaveplate consisting of a polyimide film with a film thickness of notmore than 20 μm, wherein said optical waveplate is arranged in tightcontact with an end face of said waveguide such that an opticalprincipal axis of said optical waveplate forms a certain angle with saidwaveguide substrate.
 23. A device according to claim 22, wherein saidoptical waveplate is a quarter waveplate having a product of an in-planeanisotropy of refractive index and a thickness set to a quarter of awavelength of light propagating through said optical waveguide, and anoptical principal axis of said quarter waveplate forms an angle of 45°with said waveguide substrate.
 24. A waveguide device unit comprising aplurality of waveguide devices formed on a single substrate and eachhaving an optical waveplate consisting of a polyimide film with a filmthickness of not more than 20 μm, wherein said waveguide devices arecoupled together through optical waveguides.
 25. A polarizationconverter, wherein a waveguide device comprises a single opticalwaveguide, and a quarter waveplate consisting of a polyimide film with afilm thickness of not more than 20 μm and having a refractive index anda thickness set to a quarter of a wavelength of light propagatingthrough said waveguide is inserted into a groove formed in a waveguidesubstrate so as to cross said waveguide substrate at a predeterminedangle with said waveguide.
 26. A converter according to claim 25,wherein said predetermined angle is an angle between 80° and 90°.